A last-ditch proposal from one of the Senate’s most moderate Republicans may determine whether Congress creates an independent commission to investigate the U.S. Capitol attack or the legislation to do so dies.

The Senate is expected to vote as soon as Thursday on a House-passed bill to create the 10-person panel, of which five positions, including the chairman, would be selected by Democratic congressional leaders, and five, including the vice chairman, would be selected by their Republican counterparts. The panel would have subpoena power, but only on a bipartisan basis.

Thirty-five House Republicans backed the bill last week, but in the Senate, the proposal has faced increasingly long odds as GOP senators criticize the proposed commission as partisan, redundant or otherwise objectionable and unnecessary. Many of those Republicans, including Senate Minority Leader Mitch McConnell (R-Ky.), argue Congress is doing enough already to seek accountability for the Jan. 6 riot by supporters of former president Donald Trump.

Those willing to entertain voting for the commission have focused their dissatisfaction on concerns its staffing may prove biased and worries that the investigation will be used as a political cudgel if its findings are not made public before 2022, a midterm election year. Republicans hoping for a return to power in the Senate and House are wary of an investigation that will probably prove damaging to Trump, distracting from their campaign criticism of President Biden and other Democrats.

Enter Sen. Susan Collins (R-Maine), a moderate who has expressed willingness to vote for a commission, if the House-approved legislation goes through some changes. This week, she began circulating an amendment to build Republican support for the bill, but by Wednesday evening, it remained unclear whether her proposed changes would be enough to bring the 10 GOP senators needed on board — or even whether Democrats would accept them.

Collins has proposed making all staff hires subject to the cooperation of the commission’s chair and vice chair and accelerating the timeline under which it must disband after releasing a final report — due Dec. 31 — from 60 to 30 days. But those two changes are a departure from the terms under which the independent 9/11 Commission — which has been held up as a model for the Jan. 6 investigative panel — was established and disbanded.

When asked whether Democrats could support those changes, a spokesman for Senate Majority Leader Charles E. Schumer (D-N.Y.) did not answer. Should an amended Jan. 6 commission bill pass the Senate, it would have to go back to the House for its approval. The House is not scheduled to take another vote until June 14.

But the more pressing question Wednesday was whether Collins’s gesture would be enough to rally Senate Republican votes to back the panel. By evening, that prospect was still facing challenging odds.

“An improvement,” was Sen. Richard C. Shelby’s (R-Ala.) assessment of Collins’s proffer. “But we’ll have to see, have to weigh the whole thing. I’m against [the bill] in its present form.”

At present, only two Republicans — Sens. Mitt Romney (Utah) and Lisa Murkowski (Alaska) — have declared their intention to back a commission, even without Collins’s changes. Collins told reporters Wednesday that she was “working very hard to secure Republican votes” to add to their number.

If her gambit works, it will be because of senators like Bill Cassidy (R-La.), a conservative who, during Trump’s impeachment trial earlier this year, voted to convict the former president for inciting the mob that would be the subject of this investigation. On Wednesday, Cassidy warned that if Congress fails to approve an independent commission, something more partisan would come in its place.

“There’s going to be some investigation, if it’s not the commission, it’ll be a House select committee,” Cassidy told reporters, arguing that a select committee “would be more heavily influenced toward the Democratic perspective” and would “go to 2022, absolutely.”

But it is unclear there are enough Cassidys-in-waiting to bring 10 Republican votes — the number needed to overcome a procedural filibuster.

The campaign to persuade Republican senators gained momentum Wednesday when Gladys Sicknick — mother of Capitol Police Officer Brian D. Sicknick, who suffered two strokes and died one day after confronting rioters at the Capitol — announced her desire to meet with every Republican senator to urge them to support the creation of a commission. Gladys Sicknick and Sandra Garza, the late officer’s partner for the past 11 years, intend to be on Capitol Hill on Thursday, but it is unclear how many GOP senators will grant them an audience.

It is also unclear how much sway the family of the fallen officer will have over the votes of a party that remains dominated by Trump, who has come out forcefully against the commission — and who could be forced to testify before it. Only seven Senate Republicans broke with their party to vote in favor of convicting him of the impeachment charges, and not all of them like the idea of establishing the panel.

In a statement last week, Sen. Richard Burr (R-N.C.), another of the seven, said “I don’t believe establishing a new commission is necessary or wise,” citing related, ongoing investigations. A spokeswoman for Burr said Wednesday that Collins’s proposed amendment would not change the senator’s mind.