For their part, a group of Democratic insurgents targeted incumbent lawmakers who had no whiff of scandal and reliably liberal voting records. And incumbency, corruption-free service and voting the right way did not prove sufficient for Representatives Joseph Crowley of New York and Michael P. Capuano of Massachusetts, who were upset by women of color, Alexandria Ocasio-Cortez and Ayanna Pressley, who argued that this moment demanded something more.

The question now is whether 2018 represented a Trump-era anomaly — a norm-defying president and a radicalized opposition party — or the start of a new, less genteel primary culture.

Republicans are bleeding in open seats

The Republican House majority is beleaguered, burdened by Mr. Trump’s intense unpopularity and battling an imposing set of Democratic challengers with broad appeal. But at the outset of the general election, there is no more urgent problem for the party than the dozens of open seats Republicans must defend, where long-serving incumbents chose to retire and the party has struggled to field strong replacements.

But their margin for error is painfully slim, and the booming economy is not offering them the sort of political lift they expected as Mr. Trump’s divisive style overwhelms the campaign.

[Track the key House races here.]

Democrats must gain 23 seats to take control of the House, and they could win a quarter or more just from these vacancies. Democrats might have struggled to beat Representative Dave Reichert in the Seattle suburbs or Representative Frank LoBiondo in southern New Jersey, and it could have been impossible to defeat Representative Ileana Ros-Lehtinen in Miami. But all three incumbents retired, and their seats are now tossups or leaning toward the Democrats.

There are similarly endangered open seats in most parts of the country, including rural Kansas, Southern California and the suburbs of Philadelphia. The challenge extends to the Senate, too, where Democrats have a slim-but-plausible pathway to taking control in large part because Senator Bob Corker of Tennessee decided to retire and former Gov. Phil Bredesen, a popular moderate Democrat, entered the race to replace him. (He faces Representative Marsha Blackburn.)

Republicans may yet keep their control of the House, with a powerfully funded, overwhelmingly negative campaign aimed at disqualifying Democratic challengers in swing seats. But their margin for error is painfully slim.