US President Donald Trump has vowed to jettison any attempt at bipartisanship and fight back if the new Democratic majority in the House of Representatives uses its powers to press investigations into his administration.
Trump, speaking during a combative news conference in which he trumpeted his role in the Republican gains made in Tuesday's midterm elections, warned he would adopt a 'warlike posture' 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, possible business conflicts of interest, and any links between his 2016 election campaign and Russia.
Trump was buoyed 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 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 polarisation 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.
The Democrats fell short of a tidal wave of voter support that would have won them control of both chambers of Congress.
But in the 435-member House, the party was headed for a gain of around 30 seats, beyond the 23 they needed to claim their first majority in eight years.
House Democrats could force Trump to scale back his legislative ambitions, possibly dooming his promises to fund a border wall with Mexico and pass a second major tax-cut package.
Legislators could also demand more transparency from Trump as he negotiates new trade deals with Japan and the European Union.
'Today is more than about Democrats and Republicans, it's about restoring the Constitution's checks and balances to the Trump administration,' Pelosi told supporters at a victory party on Tuesday night.
Trump also mocked Republican candidates who had refused to back his policies and ultimately lost their races, such as US Representative Barbara Comstock of Virginia.
'They did very poorly. I'm not sure that I should be happy or sad but I feel just fine about it,' he said.