These days they can be found in Pizza Huts, Panera Breads, living rooms and libraries, plotting political strategy to help Democrats wrest back power. They meet in small groups — dozens in western Pennsylvania alone — with names like Oil Region Rising, Slippery Rock Huddle, Progress PA, 412 Resistance, Indivisible Wexford. They have undergone a civics crash course, learning the intricacies of voter canvassing, candidate recruiting, database building and the often arcane rules of local politics.

Their goals have become both narrower and more ambitious: Yes, to achieve Democratic victories in this fall’s midterm elections, but also, more fundamentally, to rebuild the Democratic Party from the ground up, including in long-neglected places won by Mr. Trump. In party primaries in Pennsylvania next week, they will be focused not just on the congressional and governor races, but on their local Democratic committees, the county-level governing boards — an office so little celebrated that many of the seats have long sat vacant.

“It was not, ‘Oh my God, the Democratic Party is too far to the left or to the right,’” said Lara Putnam, a history professor at the University of Pittsburgh who has both participated in and studied the new grass-roots groups. “It was that we assumed the infrastructure was there and it’s not.”

Around Pittsburgh, the grass-roots groups have coalesced into a sprawling and potent political operation. A “super PAC” is even being formed. But farther outside the city, where the resources and activists are scarcer, the task of a political overhaul falls to anyone willing to put in the work.

Ms. Rentz was drawn away from her lifelong Republican voting habits by President Barack Obama, whom she very much liked, though she did not support him beyond casting a ballot. Neither Hillary Clinton nor Bernie Sanders inspired much fondness.

But she “hated hated hated” Mr. Trump. This was not out of objection to any particular policy, but to nearly everything else: The name-calling, the bluster, the “Access Hollywood” tape, the mocking of the disabled, the tolerance of hate groups, “the fact that he couldn’t complete a sentence.”