Sen. Jon Tester (D-MT) warned of a possible future attack on the U.S. Capitol that will have a result which will be “far worse” than the deadly January 6th riot.

Tester made the comment to Politico, which reported Thursday that Senate Republicans are prepared to filibuster a proposed independent commission to probe the riot.

“We’ve got to get to the bottom of this shit,” Tester told the news outlet. “Jesus. It’s a nonpartisan investigation of what happened. And if it’s because they’re afraid of Trump then they need to get out of office. It’s bullshit. You make tough decisions in this office or you shouldn’t be here.”

Senate Minority Leader Mitch McConnell (R-KY) recently informed fellow GOP lawmakers that he opposes the 9/11-style commission, signalling the House-passed bill could be doomed in the upper chamber.

“We’ve had a chance to hear from House leadership about what they saw in the bill. It doesn’t appear right now that they believe that it is bipartisan in nature, which to me is extremely disappointing,” Sen. Mike Rounds (R-SD) told Politico. “The way that the bill is written right now, I would feel compelled to vote against it.”

Senate Minority Whip John Thune (R-SD) confirmed to Politico that the bill does not have enough support to pass, stating: “At the moment, no.”

“The House-passed version won’t have 10,” he added.

Sen. Mitt Romney (R-UT), a staunch Trump critic who voted to impeach the former president, was the first GOP Senate to back the House bill while Sen. Lisa Murkowski (R-AK) supports the measure as well.

Texas Sen. John Cornyn (R), who once supported the idea of the commission, said he now believes Democrats are trying to use it as a political tool.

“I don’t think this is the only way to get to the bottom of what happened,” Cornyn said.

Republicans have also pointed to a bipartisan Senate report that is expected next month, saying it will be sufficient to fix security problems in the Capitol.

Four protesters died during the insurrection, including a woman who was shot and killed by police as she tried to break into the House chamber with lawmakers still inside. Dozens of the officers defending the Capitol were injured. Hundreds of people have been arrested.

The Associated Press contributed to this report.