Republicans on Monday braced for the impact of the impending indictment of former President Donald J. Trump, with his allies on Capitol Hill flexing their investigative powers to target the prosecutor pursuing Mr. Trump while the leading rival for the 2024 G.O.P. presidential nomination, Gov. Ron DeSantis of Florida, took his first swipe at Mr. Trump’s personal conduct.

Mr. Trump’s call over the weekend for his supporters to take to the streets in protest of what he described as his looming arrest left even some of his allies on the right fearful about what would come next. Memories are still fresh from Jan. 6, 2021, when Mr. Trump’s supporters stormed the Capitol in a violent riot that has since resulted in more than 1,000 arrests.

With police barricades going up outside the Criminal Courts Building in Manhattan on Monday, prominent Republicans, including Mr. Trump’s allies, were divided over whether to encourage mass protests. Some influential voices on the right urged caution and for his supporters to stay away, particularly from New York, where any potential unrest would invite prosecution from the same official who is expected to charge Mr. Trump. Others said not protesting the indictment of a former president was tantamount to ceding their constitutional rights.

“I get that there are some fears and concerns based on what happened on Jan. 6,” said Gavin Wax, the president of the New York Young Republican Club, which organized a demonstration in Manhattan on Monday evening that was sparsely attended, with the news media vastly outnumbering protesters. “But it’s ridiculous and pathetic and nihilistic to say that a conservative can’t peacefully protest.”

The day’s events represented an uneasy calm before an expected political and legal firestorm. A Manhattan grand jury is expected to soon indict Mr. Trump in connection with hush money payments that kept a pornographic actress, Stormy Daniels, from speaking out in 2016 about an affair she said she had with Mr. Trump years earlier.

Three Republican House committee chairmen made an extraordinary pre-emptive strike on Monday against the Manhattan district attorney, Alvin L. Bragg, demanding that he provide communications, documents and testimony about his investigation, a rare attempt by Congress to involve itself in an active criminal inquiry.

Referring to the expected indictment, Representatives Jim Jordan of Ohio, James R. Comer of Kentucky and Bryan Steil of Wisconsin wrote, “If these reports are accurate, your actions will erode the confidence in the evenhanded application of justice and unalterably interfere in the course of the 2024 presidential election.”

Mr. Trump’s lawyers have quietly pushed the Republican-led House to intervene. Last month, a Trump lawyer, Joseph Tacopina, wrote to Mr. Jordan calling on Congress to investigate the “egregious abuse of power” by what he called a “rogue local district attorney,” according to a copy of the letter obtained by The New York Times.

The expected indictment is already roiling the 2024 campaign trail.

In Florida, Mr. DeSantis, who had faced pressure from Trump allies to speak out against the case, broke two days of silence on Monday, joining the chorus of other Republicans who have accused Mr. Bragg of “weaponizing” his office.

But Mr. DeSantis went further. The governor, who has not yet declared his candidacy for president but is traveling the country, including to key early primary states, needled Mr. Trump over the conduct at the heart of the investigation.

“I don’t know what goes into paying hush money to a porn star to secure silence over some type of alleged affair,” Mr. DeSantis said to chuckles from the crowd at the event in Panama City, Fla. “I just, I can’t speak to that.”

“But what I can speak to,” he continued, “is that if you have a prosecutor who is ignoring crimes happening every single day in his jurisdiction, and he chooses to go back many, many years ago, to try to use something about porn star hush-money payments, you know, that’s an example of pursuing a political agenda and weaponizing the office.”

Mr. Trump fired back in his characteristically caustic and personal terms, making insinuations about the governor’s sexuality and raising questions about whether Mr. DeSantis — who is married to a woman — was inappropriately involved with students when he was a teacher in his early 20s.

“Ron DeSanctimonious will probably find out about FALSE ACCUSATIONS & FAKE STORIES sometime in the future, as he gets older, wiser, and better known, when he’s unfairly and illegally attacked by a woman, even classmates that are ‘underage’ (or possibly a man!). I’m sure he will want to fight these misfits just like I do!” Mr. Trump wrote on his social media site.

Mr. DeSantis, who has tried to avoid engaging with Mr. Trump’s intensifying attacks, has previously waved away the former president’s allegations of relationships with students. “I don’t spend my time trying to smear other Republicans,” he said last month.

Representative Elise Stefanik of New York, the highest-ranking member of the House so far to endorse Mr. Trump, predicted in an interview that the expected indictment “only strengthens President Trump moving forward.” And Mr. Trump did in fact score an endorsement from Mr. DeSantis’s home state on Monday — from Representative Anna Paulina Luna, who indicated that the expected indictment had pushed her to unequivocally choose sides.

“This is unheard-of, and Americans should see it for what it is: an abuse of power and fascist overreach of the justice system,” Ms. Luna said in a statement to The Times.

Mr. Trump has long measured the strength of his political standing by the blunt metric of the size of the crowds that show up for him, in good times and in bad. When the “Access Hollywood” video first broke in 2016, Mr. Trump found comfort in the small band of supporters who stood in solidarity with Trump signs outside Trump Tower on Fifth Avenue, visiting them briefly with a fist pump. And, once he became president, the first mini-drama of his White House tenure was related to his insistent exaggerations about the crowd size at his inauguration.

And so there was little surprise, despite the shadow of Jan. 6, that he exhorted his supporters on his social media site on Saturday to “PROTEST, PROTEST, PROTEST!!!”

On Saturday, Mr. Trump has scheduled his first large rally of the 2024 campaign in Waco, Texas — far from any courthouse. The timing coincides with the 30th anniversary of the federal government’s standoff and eventual deadly siege in Waco of a compound run by the Branch Davidian religious sect — an iconic event in right-wing, antigovernment lore.

While the rally is expected to be well attended, prominent online pro-Trump voices have not all heeded and repeated the former president’s call for supporters to mobilize at protests.

“Better to stay home,” advised Jeffrey Clark, the former Justice Department official who was nearly named acting attorney general by Mr. Trump in late 2020 as the president sought to overturn his election loss.

Some Trump allies have indulged in unfounded conspiracy theories about entrapment, claiming that the federal government would somehow infiltrate any protest to encourage violence, or that left-wing agitators would initiate violence or spur crowds toward it.

“How many Feds/Fed assets are in place to turn protest against the political arrest of Pres Trump into violence?” Representative Marjorie Taylor Greene, a Georgia Republican who is one of Mr. Trump’s close allies, wrote on Twitter.

Jesse Kelly, a syndicated right-wing radio host, said on Monday that “what’s happening to Trump is beyond injustice” but still pressed Trump supporters to stay away from any protests.

In one tweet, Mr. Kelly complained that Mr. Trump had not helped with the legal bills of those involved in Jan. 6. In another, he included a screenshot of Mr. Trump urging people to protest and captioned it, “This is abuse of his followers and I despise it.”

Late Monday, Mr. Trump tried to call into a streaming “Prayers for Trump” call co-hosted by Roger J. Stone Jr., his longest-serving confidant. As technical difficulties disrupted their connection, Mr. Stone called for people to be “peaceful,” “civil” and “legal” in their protests.

As House Republicans gathered this week in Orlando, Fla., ostensibly to plot their policy agenda and how to position themselves for the coming fiscal fights on Capitol Hill, the disruptive force that Mr. Trump remains for the party was on display, even as G.O.P. lawmakers lined up almost uniformly against his prosecution.

The Republican speaker of the House, Kevin McCarthy, who owes his post in part to Mr. Trump’s support, was among those urging Trump supporters to stay away from protests on Sunday, for instance. “I don’t think people should protest this, no,” he said.

Ms. Stefanik, one of Mr. Trump’s most fervent defenders, dissented. “I do believe people have a constitutional right of freedom of speech to speak up when they disagree,” she said.

A few hundred miles away, Mr. DeSantis was attempting his own high-wire balancing act when it comes to Mr. Trump. He criticized Mr. Bragg as “a Soros-funded prosecutor,” using the familiar language of the right to bash George Soros, the liberal billionaire philanthropist, for his indirect financial support. At the same time, Mr. DeSantis appeared to minimize the significance of a former president facing potential criminal charges.

“We’ve got so many things pending in front of the Legislature,” Mr. DeSantis said. “I’ve got to spend my time on issues that actually matter to people.”

Reporting was contributed by Alan Feuer, Luke Broadwater, Michael C. Bender and Chelsia Rose Marcius.