A background checks bill that would result in some of the most significant changes in American gun law in decades appears unlikely to pass, despite public support from a major gun rights group.

Sens. Joe Manchin, D-W.Va., and Pat Toomey, R-Pa., are coming up short on votes for their compromise legislation just hours before the Senate is to act on the measure Wednesday afternoon. Their proposal would expand gun buyer background checks to all commercial sales, closing the gun show and online sales loopholes.

A vote on the amendment is scheduled around 4 p.m. The bill needs 60 votes to overcome a filibuster threat and end debate.

Manchin remains hopeful the bill will pass but also suggests it is unlikely. The senator said 90 percent of Senate Democrats are prepared to vote yes, but the measure still needs some Republican support.

“If we had just 20 percent of our Republican colleagues, that’s not a heavy ask. It’s not a heavy lift,” Manchin said on MSNBC’s “Morning Joe.” “Only 20 percent. That’s nine members. Nine members [and] this thing would be home. And we will see.”

Shortly thereafter, his spokesman Jonathan Kott issued a statement reiterating that the senator is still optimistic.

“[He is] hopeful that if senators and the American people read the bill, they will support his commonsense approach to require criminal and mental background checks for advertised sales, including purchases at gun shows and online sales, without infringing on Americans’ Second Amendment rights,” Kott said, adding that Manchin has also has the support of an A-rated NRA member and three Republican senators. “With a record like that, I see no reason to bet against Senator Manchin today. He will continue to explain his bill to his colleagues and anyone with concerns until the minute they vote.”

The background checks measure would be the toughest gun control law the Senate has passed, if it does, because an assault weapons ban was dropped from the Democrats’ main gun control bill early in the debate for lack of support.

The Manchin-Toomey proposal is backed by the 650,000-strong pro-gun group Citizens' Committee for the Right to Keep and Bear Arms, which helped influence the bill and wrote a portion of it.

The group's chairman, Alan Gottlieb, announced support Sunday when he said the bill will advance the group's cause and make exemptions for the sale or transfer of firearms between family members.