Once a bastion of union power, rural western Pennsylvania has been veering rightward for years, a shift that went into overdrive with Mr. Trump. After the 2016 election, small Democratic groups began sprouting up all over, many started by mid- and late-career women who had done little, if any, political work before.

In Panera booths far from blue Pittsburgh, they were elated to find others who thought like them. Surely there had to be more — if not other Democrats, at least Republicans turned off by the president.

“He’s losing his vote base,” thought Christina Proctor, 42, when she joined the ranks of the newly energized in Washington County. She had been alarmed by the local fervor for Mr. Trump in the run-up to the 2016 election, but in the months that followed she thought this allegiance was flagging. She does not think that anymore. “They’re 100 percent on board,” she said.

In 2018, a strong year over all for Pennsylvania Democrats, Republicans mostly held the line in the counties outside of Pittsburgh. In last year’s local elections in some of the places that helped send Mr. Lamb to Congress — his district has since been redrawn, leaving several of those counties, including Washington County, in a more conservative congressional neighborhood — Republicans took control of one county government after another. In some places, the Republican Party is in its strongest position in nearly a century.

“I’m very pessimistic of winning anything here, anything of note,” said Jake Mihalov, a public defender in Washington, Pa., who ran for district attorney as a Democrat. He estimated that his campaign and its supporters knocked on tens of thousands of doors. He lost by more than 25 percentage points. Working toward the 2020 elections, in which he sees the only realistic goal for local Democrats as losing the presidential race a little less badly, “is going to take motivation that I don’t have right now,” he said. “It was tough when we were optimistic.”

On Wednesday evening last week, as senators in the other Washington argued about calling witnesses in the impeachment trial, the new leaders of the local Democratic committee in Washington, Pa., sat in their headquarters, debating whether they had reached the limit of people in the county who were still persuadable.

Ms. Proctor, now the party’s vice chairwoman, believed they probably had. She and the current chairman, Ben Bright, 50, had both been part of the grass-roots surge after the 2016 election, joining a group called the Washington County Democrats; Ms. Proctor would later learn this was not actually the local party. The county’s official Democratic committee was apparently not big on meeting, campaigning or advertising itself.