It was hot last Thursday. Texas hot — mid-90s and climbing fast. Near the south steps of the State Capitol, where a crowd had gathered to protest Republican legislators’ move to redraw the state’s congressional map, people were sweating through their shirts and packed into patches of shade.

Passions were running even hotter. “Are you ready to fight?” yelled Leonard Aguilar, the secretary-treasurer of the Texas A.F.L.-C.I.O., which helped organize the rally. “Damn right!” bellowed the crowd in a call-and-response.

Mid-decade redistricting is rare. But for weeks President Trump had been pressing Texas Republicans to redraw their map in order to gain five more House seats before next year’s midterm elections. Like a loyal soldier, Gov. Greg Abbott put the issue on the agenda for the Legislature’s special session, which opened on July 21. And on Wednesday, Republican lawmakers released a new map that, if approved by the Republican-controlled Legislature, would likely deliver Mr. Trump those extra seats.

But on that sizzling Thursday last week in Austin, the filing was still nearly a week away and the debate was gearing up. Inside the Capitol, a freshly formed redistricting committee was holding the first of three public hearings. Outside, a parade of speakers, including multiple Democratic lawmakers, were channeling, and stoking, rallygoers’ outrage.

This battle is part of Mr. Trump’s larger assault on democracy, warned the former Texas Democratic representative Beto O’Rourke. And a moment this perilous calls for moving “from defense to offense,” he posited. “In every single county in Texas, in every state in the Union, what if, instead of awaiting the punch thrown by the other side, we throw the punch first?”