Republicans won control of the US Senate with victories in West Virginia and Ohio on Tuesday, ensuring Donald Trump's party will control at least one chamber of Congress next year.
Republicans also posted early gains as they battled to retain control of the House of Representatives.
But Tuesday's results ensured Republicans would be able to help Trump appoint conservative judges and other government personnel if he wins the presidential race, or block much of Democrat Kamala Harris's agenda if she prevails.
Republican Jim Justice was projected to win an open Senate seat in West Virginia shortly after polls closed, taking over the seat previously held by Joe Manchin, a Democrat-turned-independent.
In Ohio, Republican Bernie Moreno was projected to defeat incumbent Democrat Sherrod Brown.
Those two victories ensured Republicans will hold at least a 51-49 majority in the Senate, with further gains possible as results in other competitive races come in.
Republicans also won several races that could allow them to expand their 220-212 majority in the House, though the final outcome may not be known for days.
They won a Democratic-held district in Pennsylvania that includes Scranton, Democratic President Joe Biden's hometown, and picked up seats from Democrats in North Carolina, where they had redrawn district lines to their advantage.
Democrats won a Republican-held seat in upstate New York and a seat in Alabama that had been redrawn to comply with a US Supreme Court order to create a Black majority district.
Democrats now need to flip at least six seats to take control of the 435-seat chamber.
In Delaware, voters made history by electing Democrat Sarah McBride, the first openly transgender member of Congress.
As in the presidential election, the outcome will likely be determined by a small slice of voters.
Fewer than 40 House races are seen as truly competitive.
Republicans stand a chance to widen their Senate majority further if they win in Montana, where Democrat Jon Tester faces a tough reelection battle, and prevail in several competitive Midwestern states.
But they are unlikely to end up with the 60-vote majority needed to advance most legislation in the chamber.
In Texas, incumbent Republican Senator Ted Cruz was projected to win reelection, holding off Democrat Colin Allred.
In Nebraska, Republican Senator Deb Fischer was holding off a surprisingly strong challenge from an independent candidate, Dan Osborn, who has not said whether he would line up with Democrats in the Senate if he were to win.
The Senate was set to see two Black women serving simultaneously for the first time, as Democrat Lisa Blunt Rochester won in Delaware, and several media outlets projected Democrat Angela Alsobrooks would win in Maryland.
HOUSE UP FOR GRABS
The outcome was still up in the air in the House. Analysts say Democrats could easily pick up enough seats to win control of the chamber, though there are no signs of a "wave" election, akin to 2018 or 2010, that would result in a decisive shift in power.
With at least 200 seats safe for each party, the winning side will likely end up with a narrow majority that could make governing difficult.
That has been evident in the past two years as Republican infighting has led to failed votes and leadership turmoil and undercut the party's efforts to cut spending and tighten immigration.
Tight races in the heavily Democratic states of New York and California could determine House control, though the final outcome may not be known for several days as California typically takes several days to count its ballots.
