(CNN) Texas Sen. John Cornyn was clear in February: Congress should establish a 9/11-style outside commission to investigate the January 6 attack, a rare sign of bipartisanship just a month after the deadly Capitol insurrection.
"I agree w/Speaker Pelosi -- a 911-type investigation is called for to help prevent this from happening again," Cornyn, a member of Senate GOP leadership, tweeted and endorsed an idea floated by the Democratic speaker.
Three months later, Cornyn has changed his tune and so have his Senate Republican colleagues, who are now poised to filibuster the proposal and prevent debate from occurring on the floor -- a vote that is likely to happen this week.
Asked what changed, Cornyn told CNN on Monday: "The process has been hijacked for political purposes. And I think that's a shame," calling on Congress to investigate instead.
Cornyn's remarks reflect the broadening opposition by top Senate Republicans to a probe amid growing fears that the investigation could become politically damaging to former President Donald Trump and some GOP members of Congress -- and give Democrats fodder to go after Republicans in the run-up to the 2022 midterms. Top Republicans are indicating they'd rather block the bill now -- and take the political hit -- than allow a sprawling investigation to go on for months at a time when they are trying to unify against President Joe Biden's agenda.
Senate Minority Whip John Thune, a South Dakota Republican, told reporters on Monday that there's concern that the probe would end up "dragging on indefinitely." And he indicated there's no path forward for the bill "in its current form."
Ten Republicans would need to break ranks in order for Democrats to get the 60 needed to overcome a filibuster. And on Monday, one GOP senator, Mitt Romney of Utah, indicated he would back the bill.
"I would support the bill," Romney said.
Sen. Susan Collins, a Republican of Maine, said Monday she has reached out to top House Democratic leaders to seek changes on how staff are chosen on the commission and to ensure the probe doesn't drag into 2022.
"I think a commission is important to -- for us to better understand what led up to January 6," Collins said Monday.
But her views are decidedly in the minority of the GOP.
"I'm against the commission," said Florida Sen. Rick Scott, who is leading the Senate GOP campaign committee in the 2022 election cycle.
Sen. Chuck Grassley, an Iowa Republican, said he would vote to block debate on the bill -- and said there needed to be a broader scope for any such probe.
"They ought to be smart enough to know that there ought to be a lot of things going around this country other than just what happened on January 6 -- and what happened in Portland and Seattle and Minneapolis and a lot of other places, including a lot of the antagonism against Jewish people right now being attacked in Los Angeles and New York," Grassley said.
Grassley added: "There's plenty of things wrong that ought to be investigated other than just January 6."
The bill, which was negotiated by a House Democrat and House Republican and was approved by a bipartisan House majority last week, would establish a 10-member commission, selected evenly by the leaders in both parties, with both sides having equal subpoena power. They would be charged with reporting their findings by year's end on the attack and the "influencing factors" behind it.
Cornyn now says that such a probe is better suited for the standing committees of Congress.
"But as I think I told you there is another way to do it, and that's use our standing committees, like we did the Russia investigation, which was bipartisan, which took three years," Cornyn said of the Senate Intelligence Committee's probe. "I'm not suggesting we take three years, so the idea of a commission I think was a good one, but if it's going to be used for political purposes, it's not our only way to get to the bottom of this."
The bipartisan legislation that passed the House was modeled after the bipartisan 9/11 Commission. But Cornyn took issue with the original Pelosi offer, which was to have more Democratic appointees than GOP commissioners, even though the speaker later backed off that idea and supported an evenly split commission.
Asked if he would be open to the bill with some changes, Cornyn said, "It's a moot issue from my perspective because we can just do this through the standing committees."
Republicans have cited an upcoming report from two Senate committees as reason enough to not have a separate commission, which they say would be duplicative.
But Senate Rules Chairwoman Amy Klobuchar, a Minnesota Democrat, told CNN that an upcoming report from two Senate committees on the January 6 attack is "not a substitute" for creating an outside commission to investigate the insurrection.
Klobuchar said the report -- which will be released the week of June 7 -- will focus on "recommendations for immediate things" that must be done with the US Capitol Police board, intelligence sharing and other steps that could taken now. She said that is much different than a broader probe conducted by an outside panel modeled after the 9/11 commission.
"We've interviewed many people since our initial two public hearings, but we want to get it out so we can make changes now," Klobuchar said Monday. "That's a different thing than with the 9/11 Commission, or what this commission would be, which is just longer-term systemic issues that led to this, and that also (includes) longer-term systemic changes."
The report is being drafted by her panel along with the Senate Homeland Security and Governmental Affairs Committee. Sen. Rob Portman, the top Republican on that committee, told CNN that the probe is narrower in scope to focus on "why was the Capitol so unprepared" that day.
"It's well-researched on evidence, findings, leading to specific recommendations," Portman said. "It's serious. But it's not meant to cover everything, which is what I think, you know, some of my Democratic colleagues had hoped for it to be more about Trump and about the motivations. It's more about what happened that day. Why we weren't better prepared, how we can get better prepared, why the response was so slow."