FILE - In this March 28, 2012 file photo, Sen. John McCain, R-Ariz. listens during a news conference on Capitol Hill in Washington. McCain said in an interview posted online Friday that "foreign money" was helping fellow Republican Mitt Romney's presidential hopes and singled out one of his ally's most generous supporters. (AP Photo/Jacquelyn Martin, File) Sen. John McCain (R-Ariz.) says he won't join in filibustering the gun-control measure. (Jacquelyn Martin/AP)

Updated at 6:40 p.m. ET.

The Senate will hold the first key procedural vote on a bill to curb gun violence Thursday as more than a half-dozen Republicans announced that they will join with Democrats to stop any attempt to block popular legislation drafted in response to a deadly shooting at a Connecticut elementary school.

The vote would formally start the the most wide-ranging and ambitious battle over gun control in 20 years.

In scheduling the vote, Senate Majority Leader Harry M. Reid (D-Nev.) said he doesn’t know if he has sufficient support to proceed to further debate on the bill. Regardless, “we’re going to vote on this anyway,” he said. “It may take a little time, but the American people deserve a vote.”

Senate procedural rules require Reid to secure at least 60 votes to move ahead with the legislation. Republican support to proceed doesn’t guarantee final passage of the bill — just that the Senate can actually begin formal debate. Getting at least 51 senators to support a final bill will prove difficult, as the politics of the issue are especially tricky for several Senate Democrats seeking reelection in 2014 in rural and Midwestern states.

If the vote to proceed is successful, the Senate is expected to spend the remainder of April debating and voting on a bill that would expand the gun background-check system, make gun trafficking a federal crime for the first time and provide $40 million in federal funding to help revamp school security programs. Senators of both parties would be permitted to introduce related amendments, including plans to establish an online portal for background checks, and to provide more federal funding for mental health programs assisting military veterans of the wars in Iraq and Afghanistan. Reid has also promised votes on proposals to ban military-style assault weapons and to limit the size of ammunition clips, but supporters expect those amendments to fail.

Meanwhile, Sen. Joe Manchin (D-W.Va.), who is seeking a bipartisan compromise on provisions related to background checks, emerged from Reid's office Tuesday evening without word of a final agreement but still hopeful that a deal was in sight.

“We’re getting there, we really are getting there. Everyone’s working,” he said.

“We’re not there yet, we’re closer than we’ve ever been,” said Sen. Charles E. Schumer (D-N.Y.), who’s been working with Manchin on changing parts of the bill regarding background checks.

Thus far, at least eight GOP senators — John McCain (Ariz.), Lindsey Graham (S.C.), Mark Kirk (Ill.), Johnny Isakson (Ga.), Tom Coburn (Okla.), Saxby Chambliss (R-Ga.), Kelly Ayotte (N.H.) and Susan Collins (Maine) — have said they will not join in a filibuster being planned by a handful of the party's more conservative senators.

Collins is most directly tied to the bill, as she is co-sponsoring the gun trafficking and school safety provisions.

“I am not going to join in a filibuster against bringing the bill to the floor as long as there is ample opportunity for amendments,” Collins told reporters. “It’s my understanding that amendments will be allowed to the bill, which is very important since there are portions of the bill that I do not support.”

Other moderate senators, however, said they are awaiting the results of ongoing bipartisan talks.

“I have to see first what amendments are going to be allowed, but filibuster just for the sake of trying to block it from a floor vote, I’m not there,” said Sen. Jeff Flake (R-Ariz.), who is also actively engaged in bipartisan talks over the nation’s immigration laws.

In a sign of how important Republican support would be for ensuring final passage of any legislation, at least two Democrats facing difficult reelection fights in 2014 — Max Baucus (Mont.) and Mark Pryor (Ark.) — said they might break with their party in opposition to the bill.

“My primary focus is the state of Montana, they’re my employers, I’m just the employee, I’m the hired hand here,” Baucus said Tuesday. “I care much more about Montana. I’m just going to wait and see what’s proposed.”

Pryor said he has advised Reid that he might join the GOP filibuster, “depending on how this plays out.”

The filibuster is being led by the new conservative wing of the GOP, including Sens. Rand Paul (R-Ky.), Mike Lee (R-Utah) and Ted Cruz (R-Tex.), whom McCain and Graham have clashed with in recent weeks. Sen. Marco Rubio (R-Fla.) and 10 other senators have signed on, including Senate Minority Leader Mitch McConnell (R-Ky.), whose involvement has lent the opposition more credibility.

In a statement Tuesday, Lee said that the filibuster threat "is an attempt to facilitate greater debate about the specific piece of legislation that will come to the Senate floor. By objecting to the motion to proceed, we guarantee that the Senate and the American people have at least three additional days to assess and evaluate exactly how this particular bill will affect the rights of law-abiding citizens and whether it will have any significant impact on crime."

Negotiations continue as the two sides try to come up with a compromise that would tip the scales enough to avoid a filibuster. Key players in that process include Manchin, Coburn and Sen. Pat Toomey (R-Pa.). Manchin will talk about that process at 5 p.m.

Democrats have 55 senators in their caucus, but Sen. Frank Lautenberg (D-N.J.) has been dealing with health issues and has been absent from the Senate. Sixty votes are needed to override a filibuster and bring the bill to a vote.

Read more from Washington Post Politics:

Bloomberg group to grade lawmakers on gun votes

Interactive: Where Congress stands on guns

The Fix: What the McConnell tape really tells us