Threats of a filibuster on gun-control proposals may go out with a whimper, as several Republican senators announced Tuesday that they'd side with Democrats and open up debate on the most ambitious gun legislation in 20 years.

Senate Majority Leader Harry Reid (D-Nev.) filed cloture on the motion Tuesday, setting up a procedural vote on Thursday that will determine whether the proposals move ahead — or whether they get stonewalled by Republican opposition before debate has even begun.

Four months after the school massacre in Newtown, Conn., the Senate technically has not even begun debate on any proposal to reduce gun violence, a source of frustration to gun-control advocates — and Reid.

"We’re going to vote Thursday," Reid told reporters. "I'm going to file cloture on the bill tonight. It would be a real slap in the face to the American people not to do something on background checks, on school safety, on illegal trafficking which everybody thinks is a good idea."

Democrats are optimistic that they now have enough support to move the proposals forward, and negate a filibuster threat driven by a trio of conservative senators: Mike Lee (Utah), Rand Paul (Ky.), and Ted Cruz (Texas). Reid acknowledged that he remained unsure if he had the 60 votes necessary to do so, but said, "We’re going to vote on this anyway."

Reports from various news outlets differed over the exact number of GOP senators who've said they'll vote in favor of opening up debate. Tallies ranged as high as 10, though all agreed that there were at least a half-dozen Republicans who have publicly said they'll vote for cloture as of Tuesday evening.

Democrats and the Independents who caucus with them hold 55 seats in the Senate. They're not all expected to vote for cloture, adding more uncertainty to how Thursday's vote will play out.

Early Tuesday, Sen. Johnny Isakson (R-Ga.) said he would not filibuster the motion. He was joined soon after by fellow Georgian Saxby Chambliss, as well as Sen. John McCain (Ariz.) and Sen. Lindsey Graham (S.C.). Other Republican senators who've reportedly said they'll vote for cloture include Mark Kirk (Ill.), Kelly Ayotte (N.H.), Susan Collins (Maine), and Dean Heller (Ariz.).

"I do not understand — it’s incomprehensible to me that we would not move forward with debate and amendments on an issue this important to the American people," McCain said, according to Talking Points Memo.

The possibility of beginning debate on a bill is a sharp reversal from Monday, when thirteen Senate Republicans sent a letter to Reid threatening a filibuster. The Senate's top Republican, Minority Leader Mitch McConnell (Ky.), threw his lot in with the filibuster crowd later that day, raising concern among Democrats that they'd be short of the votes necessary to proceed.

A successful vote on Thursday would only advance the bill to floor debate. Whether Reid can then muster the votes necessary to pass any gun legislation is still very much in doubt — just look at how much effort it takes to merely get the ball rolling. The Senate is expected to consider a number of measures, from limits on magazine capacity to universal background checks for gun purchases.

Also on Tuesday, Sen. Joe Manchin (D-W.V.) announced that he was nearing completion on a bipartisan deal with Sen. Pat Toomey (R-Pa.) that would expand background checks.