WASHINGTON — Lawmakers have repeatedly warned President Trump that firing special counsel Robert Mueller is a “red line’’ he can’t cross.

But now that the president is escalating his personal attacks on Mueller’s probe and skating closer to that line than ever before, Republicans in Congress are expressing reluctance to take specific steps to protect Mueller from Trump’s wrath.

Republicans who once cosponsored measures to make it more difficult for Trump to fire Mueller pooh-poohed their own legislation this week, saying there was no need to pass it now. The mass disinterest has Democrats and the handful of Trump critics left in the Republican Party worried that the red line was more of a suggestion than a threat.

“I always felt — and a lot of my colleagues have said privately — that that’s the red line that he can’t cross,’’ GOP Senator Jeff Flake of Arizona said Monday of firing Mueller. “But I feel we’re closer to that line and some people may be getting cold feet.’’

Flake was one of two Senate Republicans to publicly express alarm Monday. The other was Senator Bob Corker of Tennessee, who is also retiring.

Most Republicans say they believe Mueller should be allowed to finish his probe unimpeded. But a parade of Republican lawmakers pressed by reporters in the Capitol insisted there was no need to pass legislation or have Republican leadership deliver a strong warning to protect Mueller.

“I’m not about to give the president of the United States a red line,’’ said Senator John Kennedy of Louisiana.

On Tuesday, Senate Majority Leader Mitch McConnell broke his silence on the president’s attacks on Mueller, describing the special counsel as “thoroughly credible,’’ and praising Mueller’s appointment as “excellent.’’

But McConnell reiterated that it was not necessary to take steps to protect him. “I don’t think Bob Mueller is going anywhere,’’ he told reporters.

But it’s unclear if praising Mueller has the same effect as protecting him.

“A red line is not worth a whole heck of a lot if the people drawing it won’t enforce it,’’ said Stephen Vladeck, a constitutional law professor at the University of Texas at Austin.

Over the weekend, the president lashed out at Mueller on Twitter for the first time by name, saying his probe is staffed by “hardened Democrats’’ and began illegitimately.

White House lawyer Ty Cobb later released a statement saying the president does not intend to fire Mueller.

“I have zero concerns that the president or his team are going to fire Mueller,’’ said Senator Lindsey Graham of South Carolina, who has warned Trump that firing Mueller would be the “end’’ of his presidency. “Zero.’’

But Democrats, and Flake, are not reassured by Cobb’s statement, and believe Trump could move to fire Mueller at any moment. “Just a week ago he said he wasn’t going to fire Tillerson,’’ Flake pointed out. The Secretary of State Rex Tillerson was fired last week.

Two pieces of bipartisan legislation to protect Mueller that were considered by the Senate Judiciary Committee last September have since all but disappeared, with their Republican cosponsors now distancing themselves from the bills. Senator Thom Tillis, a North Carolina Republican who cosponsored the Special Counsel Integrity Act, told a CNN reporter Monday that there’s no need to pass the legislation soon. Graham said “not now’’ when asked if he wanted his bill to pass.

Tillis’ bill would allow Mueller to challenge his own firing in court if he believes he is not dismissed for “good cause,’’ as the special counsel statute mandates. Graham’s bill would require the Justice Department to seek approval from a judge to fire him. Some Republicans expressed concern that the bills are a constitutional overreach and would not be upheld in court. Flake says he is working on introducing a different measure to protect Mueller.

There is little Congress can do retroactively if Trump moves to fire Mueller, according to Vladeck, aside from ramping up their own investigations into Russian interference in the 2016 elections and any ties the Trump campaign may have had to it. House Republicans recently moved to close their investigation, over the objections of Democrats. The Senate is continuing its probe.

The White House has argued Trump has the right to fire the special counsel unilaterally, but legal scholars say he would likely need to appoint an attorney general willing to fire Mueller for him. Trump has vented about his dissatisfaction with Attorney General Jeff Sessions for recusing himself from the probe, but has not taken steps to remove him.

Firing Mueller would spark a major conflict, lawmakers say.

“I think every member of Congress should be speaking up in defense of the rule of law,’’ said Democratic Senator Ron Wyden. “Firing Bob Mueller will trigger a constitutional crisis, and my guess is you’d have more people in the streets over this than you can imagine.’’

Liz Goodwin can be reached at elizabeth.goodwin@globe.com.