The president's seeming indifference to claims of abuse infuriated Republicans, who were already confronting a surge of activism from Democratic women driven to protest, raise money and run for office because of their fervent opposition to Mr. Trump.

"This is coming, this is real," Stephen K. Bannon, Mr. Trump's former chief strategist, said recently about the female-fueled wave of liberal energy.

Mr. Trump's remarks illustrated a broader problem: Republican congressional leaders and strategists have pleaded with lawmakers and candidates to stay focused on economic growth and December's tax cuts, a message they hope will be their salvation before the elections in November. But that may be little more than fantasy in a campaign that will turn more on the president's conduct than any policy issue.

His comments on Friday, the first he had offered since images emerged of one of Mr. Porter's former wives bearing a black eye, were the culmination of a week's worth of politically ill-advised steps that suggest that the president and his lieutenants cannot stop themselves from blunting positive political momentum. By the weekend, Mr. Trump's State of the Union address, strong employment and wage figures as well as the onset of tax cuts seemed washed away by the latest White House controversy.

The frustration in the Republican political class is bursting forward.

"For members or anybody else who cares about keeping control of Congress, if you find yourself talking about anything but the middle-class tax cut, shut up and stop talking," fumed Corry Bliss, who runs the primary House Republican "super PAC," the Congressional Leadership Fund. "Any time spent on TV talking about anything but how we're helping the middle class is a waste of time and does nothing to help us win in 2018."

Republicans have grown accustomed to the president's lack of discipline and inability to reliably carry a message. But operatives overseeing the midterm effort and some lawmakers facing difficult re-elections are growing more alarmed that Mr. Trump's fixation on the Russia inquiry, personal slights and personality clashes inside and outside his White House are only encouraging his congressional and conservative news media allies to swerve off message.