Hours after news broke that special counsel Robert Mueller had impaneled a grand jury in the Justice Department’s investigation into Russian election interference, Donald Trump took the stage at a campaign-style rally in West Virginia to dismiss the scandal. “The Russian story is a total fabrication, it is just an excuse for the greatest loss in the history of American politics,” the president said, surrounded by a sea of red and white Make America Great Again hats. “That’s all it is.”

But while Trump’s jokes about Russian infiltration (“Are there any Russians here?”) drew boisterous cheers from Thursday night’s crowd, few on Capitol Hill are laughing. Earlier this week, Trump was effectively forced to sign a veto-proof bill leveling new sanctions against Russia and limiting his ability to lift them—a stunning rebuke of the White House and its Republican president. Meanwhile, a growing bipartisan group of senators are moving to protect Mueller’s investigation from Trump, who has signaled that he might fire Attorney General Jeff Sessions in order to curtail the F.B.I. probe.

Last week, Chuck Grassley, the Republican chairman of the Senate Judiciary Committee, said that he would not schedule hearings to confirm a replacement for Sessions if Trump were to dismiss the former Alabama senator from his post at the Justice Department. Now, amid concerns that Trump would try to make a recess appointment later this month, circumventing the Senate confirmation process, leaders in both parties have put their foot down. “We have some tools in our toolbox to stymie such action. We’re ready to use every single one of them,” Minority Leader Chuck Schumer said. On Thursday, the Senate confirmed that it will hold a series of “pro-forma” sessions—brief meetings that will occur every three days—that will prevent the president from making any appointments during the August recess.

As Mueller’s investigation accelerates and expands, probing Trump’s personal finances and business ties—what the president has described as a red line for his family—senators have introduced two bipartisan bills that would prevent the president from ordering the firing of Mueller arbitrarily and without cause. One proposal—from Republican Thom Tillis and Democrat Chris Coons—would allow any fired special counsel to contest their dismissal in court, after which point a three-judge panel would have two weeks to determine whether it was warranted. The other proposal—co-authored by Republican Lindsey Graham and Democrat Corey Booker—would delay any firing until after a three-judge panel determined whether there was cause.

“The Mueller situation really gave rise to our thinking about how we can address this, address the current situation,” Tillis told The Washington Post, adding that his bill would provide “a great opportunity, in perpetuity, for us to be able to communicate to the American people that actions were appropriate—or if not, then not.” And while it is assumed that Trump would push back on any legislation that would curtail his authority to fire the special counsel, Coons told the Post that there is “a broader bipartisan concern that the president may take inappropriate action to interfere with the ongoing, important work of Bob Mueller,” and said, “if the president were to fire the special counsel, the Senate might promptly take action to reappoint him.”

The emerging bipartisan effort represents a remarkable vote of no confidence in Trump’s ability to weather the F.B.I. investigation without attempting to obstruct justice. And it suggests that Republicans, who long defended the indefensible on Trump’s behalf, may have found their own red line. Perhaps most important, the shifting tone on Capitol Hill comes as the president’s popularity has continued to sag, threatening the party’s hold on Congress. Six months into his term, Trump has no substantive legislative accomplishments to speak of and his approval rating is as low as 36 percent. And while Trump has retained the support of his Fox News base, the G.O.P.’s various health-care bills were all widely unpopular. Another recent poll found that 64 percent of voters believe Mueller will conduct a fair investigation into whether Trump’s campaign colluded with the Russian government.

Republicans used to be afraid of Trump. Now, they’re mostly ignoring him. The administration’s ham-handed attempts to coerce Senators Dean Heller and Lisa Murkowski to vote “yes” on Obamacare repeal were met with sharp criticism from Republican lawmakers. And Trump’s threats in the aftermath of the failed health-care vote last week fell on deaf ears. “If that’s what he wants to do, he ought to just do it,” Senator Bob Corker said of Trump’s call to stop insurance payments to members of Congress. “[As] probably he’s learned over the last couple of weeks, threats don’t really go over particularly well. My advice would be to either do it or don’t do it.”