Whether Democrats or Republicans control the US Senate in January rests on three states: Arizona, Nevada and Georgia.
Races in Arizona and Nevada remained too close to call Thursday, with about 600,000 ballots yet to be counted. If the two parties split Arizona and Nevada, then a runoff in Georgia in December will decide control of the Senate.
Meanwhile, while control of the House remains in limbo, House Minority Leader Kevin McCarthy of California has started gathering support for his bid to be the next speaker.
Even if Democrats manage to keep hold of the Senate, a Republican-controlled House could disrupt President Joe Biden's ambitious legislative agenda and presage a new era of gridlock.
Biden appeared Thursday at a political event in Washington, where he praised the strong night for Democrats, saying, "This is the vote that showed some real enthusiasm for what we're fighting for."
If Democrats win both Western states, the Georgia runoff on Dec 6 will decide if they continue controlling a 50-50 Senate with Vice-President Kamala Harris breaking tie votes.
Neither Democratic Senator Raphael Warnock, a Democrat, nor his Republican challenger, Herschel Walker, cleared the 50 percent threshold needed to win outright on Tuesday. Warnock had 49.6 percent of the vote to Walker's 48.3 percent, according to CNN.
As of 3:30 pm ET, Warnock had 1,948,422 votes, while Walker had 1,899,678. The Libertarian candidate for Senate in Georgia, Chase Oliver, had about 2.1 percent, or 81,187 votes. He will not be in the runoff election.
On Thursday afternoon, Arizona GOP candidate for Senate Blake Masters seemed to be hinting at a Republican plan to challenge the results. Masters was down five points to Democratic Senator Mark Kelly and will need about 60 percent of the uncounted votes to win.
In the race for US Senate in Nevada, Republican Adam Laxalt held a slight lead over Democratic incumbent Catherine Cortez Masto, according to CNN. Laxalt had 437,018 votes, or 49.4 percent, to 421,151, or 47.6 percent for Cortez Masto, with 84 percent of votes counted as of 4:30 pm ET Thursday.
But mail-in ballots still coming in from Clark County, home to Las Vegas, and Washoe County, where Reno sits, are trending for Cortez Masto about 2 to 1, according to The New York Times.
In the race for Arizona governor, former television news anchor Kari Lake was about half a point behind Democratic Secretary of State Katie Hobbs. The Republican candidate for attorney general also trailed narrowly.
Lake said that if she wins, she will call lawmakers into a special session to make changes to the state's election laws. She wants to reduce early and mail voting and to count all ballots by hand. Lake based her campaign largely on challenges to the results of the 2020 presidential election in Arizona, which were unsuccessful.