Most House Republicans operate under an unspoken but ironclad rule: Do whatever you can to avoid provoking the wrath of former President Donald J. Trump.

But on a recent weekend here in Iowa, just days before the state’s first-in-the-nation nominating contest, two of Congress’s staunchest conservatives were doing just that as they crisscrossed the state with Gov. Ron DeSantis of Florida to make the case for a different party standard-bearer.

At stop after stop on a string of frigid, gray days, Representatives Chip Roy of Texas and Thomas Massie of Kentucky packed into crowded sports bars and coffee shops, casting Mr. DeSantis as a leader with a proven track record of conservative victories. In doing so, they issued a surprisingly blunt review of what they argued were a string of policy failures by the former president — including his inability to repeal and replace the Affordable Care Act, to complete a wall on the U.S. border with Mexico and to rein in the skyrocketing national debt — and an implicit critique of his character.

“The primary reason that I’m supporting Gov. Ron DeSantis for president is that I want my son and my daughter to be able to look up to the occupant of the Oval Office,” Mr. Roy told a packed room of caucusgoers at a sports bar in Ankeny. “Someone they can emulate. Someone that you would be proud to have them follow and look to as a leader.”

Mr. Roy and Mr. Massie have always cut singular figures in Congress. Mr. Roy, a former chief of staff to Senator Ted Cruz of Texas, has emerged as arguably the most influential conservative voice on policy in the House G.O.P. conference. Mr. Massie, a libertarian who is by turns thoughtful and mischievous, forced Congress to return to Washington to take a recorded vote on the $2 trillion stimulus measure at the height of the pandemic.

But their commitment to break with a vast majority of their colleagues — including the entire House Republican leadership — and campaign for Mr. DeSantis even as he lags badly in polling behind Mr. Trump is perhaps one of their most fraught political moves yet.

Despite the possible political risks, they have remained defiant, not only criticizing Mr. Trump’s record — even throwing in an impersonation or two of the famously thin-skinned Republican front-runner — but also openly lamenting the choices of their Republican colleagues who have endorsed him.

“I would say a good number of the people who have endorsed Trump in Congress have done it because they genuinely want him to be the president and they prefer him,” Mr. Massie said in an interview before an event with Mr. DeSantis at a sports bar in Grimes. “But a majority of them are scared of their own constituents. Not necessarily scared of Trump, but that he would rile up their constituents and that they might lose a primary. And that is disappointing to me.”

“It is a political risk,” Mr. Massie conceded of his backing for Mr. DeSantis, noting that two primary opponents had recently filed to challenge him.

He and Mr. Roy are two of just five sitting members of Congress, including Representative Bob Good of Virginia, the newly elected chairman of the House Freedom Caucus, who have endorsed the Florida governor. As of last week, every House G.O.P. leader had lined up behind Mr. Trump.

Asked about the potential political risks of campaigning against Mr. Trump, Mr. Roy replied, “I don’t give a crap what some influencers say on Twitter.”

“Frankly,” he continued, “all too many of my Republican colleagues operate out of fear. I fear the Lord. I don’t fear politics. I don’t fear political retribution. Worst-case scenario, I end up back in Texas? My life is pretty good.”

If Mr. Trump’s retinue of surrogates are MAGA darlings, Mr. DeSantis’s coterie is more of a wonky conservative clique. On the campaign trail, Mr. Massie describes how when Mr. DeSantis was in Congress, the two of them would eschew black-tie dinners in favor of reading the text of the bills they were voting on over sandwiches. Mr. Roy rhapsodizes about Florida’s balanced budget, contrasting it with the climbing national debt Mr. Trump left behind.

Their critiques begin tactfully. Mr. Roy notes that he supported Mr. Trump in 2016 and 2020, points out that many of his “great friends” worked in the Trump administration and frequently punctuates his references to the former president “with all due respect.” Mr. Massie has acknowledged that “Trump did a lot of good things.”

They are not household names, yet many voters in Iowa greeted them enthusiastically. At a coffee shop, one man boisterously cheered for Mr. Roy by name, and at another stop, a woman stopped Mr. Massie to ask if her young daughter could shake his hand.

The Trump campaign has signaled that it is unimpressed by Mr. DeSantis’s appearances with members of Congress.

“Wow, what a game changer (insert sarcasm here),” Steven Cheung, a Trump spokesman, said in a text message.

But they do appear to have personally riled Mr. Trump himself.

“Has any smart and energetic Republican in the Great State of Texas decided to run in the Primary against RINO Congressman Chip Roy,” Mr. Trump wrote last month on his social media platform, using the acronym for “Republican in name only.” “For the right person, he is very beatable. If interested let me know!!!”

In fact, the window to file candidacy for challenging Mr. Roy had already closed.

Both Mr. Roy and Mr. Massie have defied the former president before. They led a letter in the days before Jan. 6, 2021, arguing that the Constitution did not vest Congress with the authority to overturn President Biden’s electoral victory.

“Both Thomas and I, two years ago, they said, ‘Oh you’re going to be in trouble because you voted for the electors,’” Mr. Roy said in an interview. “I got more votes than anybody else in Texas in my district. I will go and honestly speak to my constituents. They know what I’m fighting for.”

Mr. Massie has already successfully fended off a primary challenge from his right: “I have the Trump antibodies. I’ve been attacked, built a resistance in my district.”

He frequently recounts to voters how Mr. Trump tried to cast him out of the party after he forced Congress to return to Washington for an in-person vote on the first coronavirus stimulus measure.

Mimicking Mr. Trump’s distinctive way of speaking, Mr. Massie regaled crowds on a recent Saturday with an impression of the call he received from the president at the time. “‘I’m more popular in Kentucky than you, and you know it,’” he recalled Mr. Trump saying. “He was! I had done polling. And he says, ‘I’m backing your primary challenger and you’re going to lose.’”

Mr. Massie went on to win by more than 60 points.

Still, there are some limits to their criticism.

When Mr. Massie reminded voters at the bar in Ankeny that as president, Mr. Trump had signed a huge bill to fund the government and promised to never again approve one single, large spending measure, one man cried out, “He lied!”

Mr. Massie paused and flashed an ironic grin: “He … misled very tragically.”

Nicholas Nehamas contributed reporting.

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