President Donald Trump on Wednesday forced Attorney General Jeff Sessions out of office a day after mid-term elections, and vowed to fight if the US House of Representatives' new Democratic majority launches probes into his administration.

Sessions — an early Trump supporter who ran afoul of the president by recusing himself from an investigation into Russian meddling in the 2016 campaign for the White House — said in a letter to Trump that he was submitting his resignation "at your request."

The 71-year-old former US senator was informed on Wednesday morning by White House Chief of Staff John Kelly in a phone call that it was time to go, according to an aide.

Sessions' departure was the first in what could be a string of high-profile exits as Trump reshapes his team to gird for his own 2020 re-election effort. The Republican president named Sessions' chief of staff, Matthew Whitaker, as acting attorney general and said he would nominate someone permanent for the job soon.

Deputy Attorney General Rod Rosenstein was visiting the White House on Wednesday afternoon for what was described by an administration official as a regularly scheduled meeting.

Nancy Pelosi, the House Democratic leader who could be the next speaker, called Sessions' ouster a "blatant attempt" to undermine the Russia probe and urged Whitaker to recuse himself from any involvement in a statement posted to Twitter.

During an often combative news conference with reporters, Trump highlighted his role in Republican gains in Tuesday's midterm congressional and gubernatorial elections, and warned of a "warlike posture" in Washington if Democrats investigated him.

Democrats will now head House committees that can probe the president's tax returns, which he has refused to turn over since he was a candidate, possible business conflicts of interest and any links between his 2016 campaign and Russia, a matter being investigated by US Special Counsel Robert Mueller.

Mueller is overseen by Rosenstein, who reports to Sessions.

Trump said he could fire Mueller if he wanted but was hesitant to take that step. "I could fire everybody right now, but I don't want to stop it, because politically I don't like stopping it," he said.

Moscow denies meddling. Trump, calling the Mueller probe a witch hunt, has repeatedly said there was no collusion.

Trump was buoyed on Wednesday by victories that added to the Republican majority in the US Senate, telling reporters at the White House that the gains outweighed the Democrats' takeover of the House. He added that he was willing to work with Democrats on key priorities but felt any House investigations of his administration would hurt prospects for bipartisanship.

"They can play that game, but we can play it better," Trump said of the possibility of Democratic investigations. "All you're going to do is end up in back and forth and back and forth, and two years is going to go up and we won't have done a thing."

The divided power in Congress combined with Trump's expansive view of executive power could herald even deeper political polarization and legislative gridlock in Washington.

There may be some room, however, for Trump and Democrats to work together on issues with bipartisan support such as a package to improve infrastructure, protections against prescription drug price increases and in the push to rebalance trade with China.

"It really could be a beautiful bipartisan situation," Trump said.

He said Pelosi had expressed to him in a phone call a desire to work together. With Democrats mulling whether to stick with Pelosi, who was speaker when the party last controlled the House, or go in a new direction, Trump wrote in a tweet earlier that she deserves to be chosen for the position.

Pelosi, at a Capitol Hill news conference before news of Sessions' firing, said Democrats would be willing to work with Trump where possible. But she added, "We have a constitutional responsibility to have oversight."

REUTERS