Democrats will keep control of the Senate, NBC News projects

Democrats will hold their razor-thin majority in the U.S. Senate, NBC News projects, staving off a full-bore effort by Republicans to leverage economic volatility and public discontent into control of the upper chamber of Congress.

The party will hold at least 50 seats in the Senate in the next Congress, after incumbents held their ground in key races and Democratic Lt. Gov. John Fetterman flipped Pennsylvania's GOP-held seat. One uncalled race, where Democratic Sen. Raphael Warnock of Georgia is defending his seat against Republican Herschel Walker, will be decided in a Dec. 6 runoff. Democrats currently control the Senate split 50-50 by party through Vice President Kamala Harris' tiebreaking vote.

While the GOP held some key advantages over Democrats throughout the cycle, analysts considered the battle for the Senate to be a virtual toss-up heading into Election Day. Incumbent Sens. Mark Kelly of Arizona and Catherine Cortez Masto of Nevada both prevailed in their closely contested races, NBC projected after days of counting in both states, clinching the chamber for Democrats.

In a tweet, Senate Majority Leader Chuck Schumer called the election results "a victory and vindication for Democrats."

Republicans had hoped, and many had openly anticipated, a "red wave" that would wash Democrats out of their majorities in both branches of the legislature. A flip in congressional leadership would have threatened President Joe Biden's legislative agenda and his ability to advance key nominations for his next two years in office.

But that wave never materialized. Democratic candidates up and down the ballot outperformed expectations from many analysts who predicted that Biden's unpopularity, coupled with historical electoral trends and persistently high inflation, could yield a rout for the party in power.

Senate Democrats will instead hold their majority — and could even add to it if Warnock defeats Walker. It gives the party another check against the GOP if Republicans flip control of the House.

NBC News has not yet projected House control as states continue to count votes in tight races.

NBC estimates Republicans could win 219 House seats once all uncalled races are settled — barely enough for a majority — while Democrats could win 216. The projection carries a margin of error of plus-or-minus four seats.