Here are the top Senate races to watch in the 2022 midterms

The battle for control of the U.S. Senate could hinge on a single race.

A handful of make-or-break Senate elections in key states are only growing more competitive as candidates barrel toward the Nov. 8 midterm elections. Only a few contests could determine how the Senate handles policy — and President Joe Biden's nominees for key posts — for the rest of the president's first term.

Republicans were considered favorites to win control of the upper chamber of Congress heading into the year. Democrats now appear slightly more likely to hold the majority in January.

Democrats hold the slimmest possible majority in the Senate, a reality that at times has reined in Biden's most ambitious policy goals. The chamber is split 50-50, and Vice President Kamala Harris holds the tie-breaking vote.

Biden's presence in the White House could make his party's bid to hold the Senate more difficult: the president's party historically struggles in midterm elections. While Biden's approval rating has climbed from the lowest points of his term, most voters polled still disapprove of the job he is doing as Americans struggle with high inflation, among other issues.

Biden's party reshaped its national campaign messaging after the Supreme Court overturned longstanding abortion rights in June. The court's contentious and unpopular ruling seemed to upend the midterms overnight, handing Democrats a powerful tool to stoke turnout among voters outraged at the conservative court and its supporters in government.

In another potential boon for Democrats, a handful of Republicans saddled with political baggage, but backed by former President Donald Trump, won their party's primaries over more mainstream alternatives. Democrats in multiple key races now face Republicans with little or no political experience, extreme policy views or scandalous personal lives, all of which could make them less appealing to general-election voters.

Of the 34 seats up for grabs in the midterms, here are the ones that could decide Senate control:

One of the most competitive and closely-watched races of the cycle pits Democratic Lt. Gov. Fetterman against Trump-backed Republican celebrity Dr. Mehmet Oz. They are vying for the open Senate seat being vacated by retiring GOP Sen. Pat Toomey.

Fetterman, the progressive former mayor of Braddock, has led Oz in the polls even after a May stroke took him off the campaign trail for months. The 6-foot-8 former college football player's towering stature and dressed-down look have aided his campaign's efforts to contrast him from Oz, a multimillionaire TV star whom it has attacked as an out-of-touch elitist and carpetbagger from New Jersey.

The Oz campaign, aided by tens of millions of dollars from Senate GOP Leader Mitch McConnell's PAC, has bombarded Fetterman with ads accusing him of being soft on crime. Oz and his staffers have also sowed doubts about Fetterman's health as he recovers from his stroke, and have hammered him for refusing to agree to more debates beyond one 60-minute face-off set for late October.

That move may be paying off: While Fetterman still leads Oz, the gap between the two candidates has shrunk considerably in recent weeks, polling averages show.

Incumbent Democratic Sen. Raphael Warnock appears to be in a tight race with Republican challenger Herschel Walker, a famed former football pro and business owner backed by Trump. 

Warnock, a Baptist pastor whose special-election victory in 2021 helped Democrats seize a razor-thin Senate majority, is fighting to win a full term representing the swing state. 

Gaffes and scandals have hampered Walker's run. He first was forced to reveal having more children than he previously acknowledged. Most recently, an ex-girlfriend accused Walker — a staunch abortion opponent — of paying for her abortion in 2009 and pressuring her to get another two years later. Walker has denied the allegation.

Polling averages of the race nevertheless show the two candidates neck and neck, with Warnock holding a slight lead. National political dynamics could be playing an outsize role. Though Biden narrowly beat Trump in the Peach State in 2020, a Monmouth University poll conducted in September showed Georgia voters viewed Trump and the Republican Party more favorably than Walker, whereas Warnock scored higher than Biden and the Democratic Party.

Polls suggest Arizona's Democratic Sen. Mark Kelly is poised to fend off a challenge from Republican Blake Masters, a venture capitalist backed by billionaire political activist Peter Thiel.

Masters, also backed by Trump, has struggled to gain a competitive footing in the swing state that Biden narrowly won in 2020. As a primary candidate, Masters echoed Trump's false claims of a rigged 2020 election and took a vocal anti-abortion stance, but he has downplayed those views in the general election. He has sought to puncture Kelly's moderate campaign messaging and tie him to Biden, who is unpopular in Arizona. 

The incumbent Sen. Kelly, a former astronaut and husband of retired Arizona Rep. Gabby Giffords, has labeled Masters a dangerous radical. Kelly has far outraised Masters: The incumbent's campaign has taken in over $52 million and held nearly $25 million in cash on hand as of mid-July, while Masters' campaign held about $1.5 million in late September, according to data compiled by OpenSecrets. 

Thiel, Masters' largest outside donor who spent $15 million on the GOP candidate before the primary election, has not donated to the PAC backing Masters during the general.

Republican political scion Adam Laxalt is challenging freshman Democratic Sen. Cortez Masto, whose Nevada seat is considered one of the GOP's best chances to flip a blue seat red.

Cortez Masto, the state's first Latina senator, has outraised and outspent her opponent. She has also received more support from PACs and other outside groups. The Nevada election is the second-most-expensive Senate fight in the nation, trailing only the Georgia race, according to ad-tracker AdImpact.

But polling averages show Cortez Masto in a virtual dead heat with Laxalt, the former state attorney general backed by Trump. Laxalt has pushed a tough-on-crime campaign message and picked up endorsements from police organizations, while echoing Trump's recent attacks on the FBI and his false claims about election fraud in 2020. Cortez Masto, who served two terms as Nevada AG before Laxalt, has also netted support from police groups.

The race to succeed retiring Republican Sen. Richard Burr of North Carolina hasn't drummed up the same level of publicity –  or spending – as some of the flashier marquee Senate fights. But GOP Rep. Ted Budd and Democrat Cheri Beasley are locked in a tight contest that could shape the final Senate map.

Beasley, the first Black woman to serve as chief justice of the North Carolina Supreme Court, is taking on Budd, a Trump-backed three-term congressman with a staunchly conservative record. The two are running side-by-side in the polls. Still, some see the Tar Heel State, which has not elected a Democratic senator since 2008, as more favorable terrain for the Republican.

Budd's record has skewed more conservative than his would-be Senate colleague, GOP Sen. Thom Tillis, on key issues such as gun reform and same-sex marriage. Beasley, meanwhile, has kept some distance between herself and Biden, saying at a recent debate that his administration could do more to fight inflation. The two candidates have also clashed over abortion rights.

Campaign finance records show Beasley's campaign has outraised Budd's. Her campaign reportedly raised a $13.3 million haul in the third quarter, nearly doubling its second-quarter fundraising. But the McConnell-linked Senate Leadership Fund PAC has spent more than $20 million opposing her. The Senate Majority PAC, aligned with Majority Leader Chuck Schumer, D-N.Y., said in mid-October that it will put $4 million more into the North Carolina race, bringing its total spending to $10.5 million.

Democrats see Republican Sen. Ron Johnson as a vulnerable incumbent. But the two-term Wisconsin senator nevertheless appears to be gaining momentum against his Democratic rival, Lt. Gov. Mandela Barnes.

The 67-year-old Johnson has scored negative approval ratings in recent polls. He has come under fire for repeatedly sowing doubts about Covid-19 vaccines, downplaying the Jan. 6 Capitol riot and being linked to an alleged "fake electors" scheme to challenge the 2020 election results, among other controversies.

Barnes, who at 35 is just over half Johnson's age, would become Wisconsin's first Black senator if elected. Johnson's campaign has slammed his opponent as being too far left for the Badger State — especially on crime, a main feature of the GOP playbook. Barnes has espoused progressive views in office, but his Senate campaign has focused more on pocketbook issues such as inflation and taxes.

Johnson is the only Republican senator running for reelection in a state that Biden won in 2020. But polling averages as of late September show Johnson pulling ahead of Barnes.

Trump-backed Republican J.D. Vance appears locked in a tight Senate race against Democratic Rep. Tim Ryan, despite running in a red-leaning state that Trump won handily in 2020.

Vance and Ryan, both Ohio natives, are competing for the seat being vacated by retiring GOP Sen. Rob Portman. Ryan, who ran a failed presidential bid in the 2020, has campaigned on economic issues and touted his connections to the state, while tarring Vance as a "San Francisco fraud." Ryan has also slammed Vance over his stance on abortion, a top concern among Democratic voters.

Vance, the author of the memoir "Hillbilly Elegy," has portrayed himself as a political outsider with a populist message. His campaign has sought to define Ryan as an extension of Biden and Democratic House Speaker Nancy Pelosi.

Vance is also a venture capitalist linked to Peter Thiel. The billionaire conservative activist boost Vance's winning primary campaign with $15 million in multiple donations to the super PAC Protect Ohio Values.

But his campaign's fundraising efforts have lagged behind Ryan's. The Democrat has $21.5 million versus Vance's $3.6 million, according to the most recent data from OpenSecrets.

While New Hampshire has voted for Democrats in every presidential race since 2000, it is considered a purple state that has recently elected Republican senators and could do so again in November. 

Incumbent Democratic Sen. Maggie Hassan's approval ratings have fallen, including among unaffiliated voters who make up a major chunk of the state's electorate.

But she holds advantages over her Republican opponent, Don Bolduc, a retired U.S. Army brigadier general who has garnered praise from Trump. Bolduc had echoed Trump's false claims of a rigged 2020 election, but reversed himself right after his primary win, saying he had concluded that the election was not stolen after "a lot of research."

The New Hampshire primary was held Sept. 13, making it one of the latest nominating contests in the country. The late election gave Hassan, who was all but guaranteed to win the Democratic nomination, a significant head start. 

Hassan's campaign fundraising has dwarfed Bolduc's. Polls show the incumbent has a sizable lead over her GOP challenger.

Former Orlando police chief and Democratic Rep. Val Demings aims to unseat incumbent Republican Sen. Marco Rubio. 

Polls put Rubio ahead of Demings, and forecaster Cook Political Report has given the race a "Lean Republican" rating.

Demings' credentials have served to counter the soft-on-crime accusations common in Republican campaigns. She has distanced herself from Biden as she competes to serve Trump's home state.

But it may not be enough to win over the Sunshine State, which voted for Trump in 2020.

Democrats are favored to prevail in Colorado, where incumbent Sen. Michael Bennet has held a steady lead over his Republican challenger Joe O'Dea, who runs a construction company. The state's voter registration trends in recent years have also skewed in Democrats' favor, a shift that resulted in Biden beating Trump by almost 14 percentage points in 2020.

But O'Dea's willingness to break with the GOP on some key issues, including abortion, has helped him project a more moderate look than many of his Trump-backed equivalents in other Senate races.

It was O'Dea's primary win that pushed Colorado onto the list of midterm races to watch. Democrats wanted state Rep. Ron Hanks, an abortion hard-liner and Trump ally, to win the GOP primary. A Schumer-aligned PAC even spent money aimed at boosting Hanks over O'Dea.

— Graphics created by CNBC's Gabriel Cortes.