•The party's majority in the House of Representatives is razor-thin, but it is enough to stall President Joe Biden's agenda for the next two years.
Republicans have secured the 218 seats needed to win a majority in the lower chamber of the Congress, the BBC's US partner CBS News projects.
The party's majority in the House of Representatives is razor-thin, but it is enough to stall President Joe Biden's agenda for the next two years.
It marks a return to divided government, with Democrats still retaining power in the Senate.
Meanwhile, former President Donald Trump has announced he will run again.
Split control of Washington was ushered in by last week's midterm elections, in which the Republicans - who had hoped to win back control of both chambers - underperformed expectations.
The Republican party is now projected to win between 218-223 seats in the 435-seat House, according to CBS.
But with votes in cliff-hanger races still being tallied, the party's majority may not be clear for days or even weeks.
Kevin McCarthy, who was elected by rank-and-file Republicans to be their nominee to replace Democrat Nancy Pelosi as the next Speaker of the House on Tuesday, celebrated having "officially flipped" the chamber.
"Americans are ready for a new direction, and House Republicans are ready to deliver," the California congressman tweeted on Wednesday night.
In order to be elected Speaker, the House Republican minority leader must work to receive majority support from the 435 members of the full House.
President Joe Biden congratulated Mr McCarthy and offered to work with Republicans to deliver results for Americans.
"As I said last week, the future is too promising to be trapped in political warfare," said the Democratic president.
"The American people want us to get things done for them. They want us to focus on the issues that matter to them and on making their lives better."
As the majority party, Republicans will run key committees, empowering them to initiate legislation and investigate Mr Biden, his family and his administration.
But any Republican-backed legislation that emerges in the House could well be blocked in the Senate, which Democrats still control.
Republicans had hoped that Mr Biden's relatively low popularity, stubborn inflation, and the fact that congressional maps were redrawn by Republican-led state legislatures would herald sweeping midterms victories.
The blame for last Tuesday's showing has largely landed on two party leaders: Mr Trump and Senate minority leader Mitch McConnell.
On Tuesday night, the ex-president formally announced a third bid for the White House in 2024 from a ballroom at his Mar-a-Lago estate in Florida.
He said voters had "not yet realised the full extent and gravity of the pain our nation is going through", but would change their minds over the next two years as he predicted things would get worse.
In Washington on Wednesday, Mr McConnell was re-elected as Senate minority leader, fending off a challenge from fellow Republican Rick Scott of Florida.
This was the first challenge to his lengthy leadership tenure in 15 years.
The two men butted heads frequently in the lead-up to the midterms and Mr Scott had said he was "not satisfied with the status quo".
The new Congress will be convened on 3 January 2023.