Republicans have their own high stakes in November. Losing control of Congress, they say, could mean a highly politicized impeachment of their president. Worse still, a rising tide for Democrats in 2018 and 2020 could put the party in control of the redrawing of House district lines after the next census.

Congress has long been polarized. Republicans complained bitterly of being frozen out of the big legislative pushes of the early Obama administration, not only the Affordable Care Act, but also the Dodd-Frank financial services law and other measures. It was Senator Harry Reid, the Democratic leader at the time, who first used the so-called nuclear option to end filibusters for administration nominees and most judicial ones.

But Ross Baker, a political scientist at Rutgers University, sees something new in the level of vitriol and hyperbole.

“It’s something that I think has only become more intense, more conspicuous, because of the outsize personality and idiosyncrasies of President Trump,” Mr. Baker said. “It has made Democrats feel that they are under a very heavy obligation to defend the norms and the institution. Republicans feel that Congress was elected with a mandate to bring about change, so what had been a kind of pre-existing edgy relationship has simply gone viral.”

For most of the Obama presidency, Republican leaders were vexed by a dwindling center and an expanding group of hard-right lawmakers who would accept no negotiations with the Democratic president. That impediment to compromise has now been joined by a similar dynamic in the Democratic Party, where a visceral hatred of Mr. Trump on the left has empowered Democratic lawmakers to refuse to deal with the Republican president. The divide has been deepened as a half-dozen Democratic senators consider a White House run in 2020.

If temperatures are to cool, the next few weeks could prove pivotal. Senator Mitch McConnell, the Republican leader, has promised a “fair and open” debate on immigration, while a new bipartisan coalition has emerged in the Senate to try to break a logjam. Senator Jon Tester, Democrat of Montana, saw something hopeful in that development, a new willingness to go around the parties’ feuding leaders.