Scores of people from across Texas packed into the State Capitol in Austin on Thursday to testify against a Republican plan, pushed by President Trump, to redraw congressional districts and protect the party’s slender majority in the U.S. House.

The public hearing, before a 21-member select committee of the Texas House on redistricting, was the first since Gov. Greg Abbott directed state lawmakers to redraw congressional lines during a 30-day special session of the Legislature.

No maps with new district lines have been publicly proposed yet by Republican leaders in the Legislature, and none were expected before the initial public hearings. Two more such hearings are set for Houston on Saturday and the Dallas area next Tuesday.

So those testifying on Thursday were left to comment on the idea of a rare mid-decade redistricting, and Mr. Trump’s stated wish that Texas Republicans should produce maps that create five additional Republican seats ahead of the 2026 midterm elections. The state has 38 congressional seats, 25 of which are currently held by Republicans.

“I am dismayed that the legislative session is not focused, first and foremost, on flood relief,” said Robin Peeples, a software designer from Temple, Texas. She added that redistricting “should not be shoved through so quickly at a time when we’re dealing with a disaster in the state.”

State Representative Jon Rosenthal, the top Democrat on the committee, responded, “I think you are the most important person we need to listen to.”

Among those who testified were Representatives Joaquin Castro of San Antonio, Greg Casar of Austin, and Sylvia Garcia of Houston, all Democrats, and each with a district that could be dramatically redrawn.

“You all are being used,” Mr. Castro told the Republicans sitting in the hearing room. “You’re being used by the White House and by Donald Trump.” Mr. Castro added that it was “telling that there are no Republican members of Congress here today to testify in support of this effort.”

More than 170 people registered to speak at the hearing, which was slated to include five hours of testimony.

All the witnesses were in some sense attacking blindly. The chairman of the redistricting committee, Representative Cody Vasut, a Gulf Coast Republican, said that even he had not seen any proposed maps.

And redistricting ahead of midterms holds risks for Republican lawmakers who could face considerable headwinds. Some G.O.P. members of Congress from Texas have worried privately that any new lines that shift Republican voters into neighboring Democratic districts could dilute Republican strength in their districts. But they have mostly avoided commenting publicly about it.

For most of the hearing, Democratic State House members asked the questions while their Republican counterparts sat mostly in silence.

Many of those who filed inside came directly from a rally outside the Capitol that was held by Democrats, including Mr. Casar and Beto O’Rourke, a former El Paso congressman and candidate for Senate and governor. Some in the crowd held signs reading “Fight the Trump takeover” and “Stop the steal.”

Republicans in other states might also look at their maps if Texas succeeds. The White House is already prodding Missouri to erase one of its two Democratic House seats.

“Texas is the testing ground for this strategy,” said State Representative James Talarico, an Austin Democrat, speaking into a bullhorn to address the crowd that sprawled onto the grass.

State Representative Gina Hinojosa from the Austin area vowed to join fellow Democratic members in a walkout to stop the redistricting proceedings. Party leaders are considering that step, but it would require the participation of 51 of the 62 Texas House Democrats.

“We did not ask to be in this moment,” she told the crowd. “We have no choice but to fight with everything we’ve got.”

After a Democrat walkout during a 2021 redistricting fight, Republican lawmakers took steps to prevent future quorum-breaking walkouts, adopting rules that would impose fines of $500 a day for each member who takes part. The attorney general, Ken Paxton, vowed that his office would assist in “hunting down” any Democrats who try to walk out.

Mr. O’Rourke urged Democrats to “throw the punch first,” suggesting that Democratic lawmakers in Texas should walk out, and that Democratic leaders in blue states should engage in their own aggressive redistricting to create more Democratic districts.

In truth, options in such states are extremely limited by state laws, independent redistricting commissions and previous gerrymandering.