Senate Majority Leader Harry Reid, D-Nev., announced Tuesday that he will hold a test vote on a gun package, as negotiators neared an agreement on background check language that would be the legislative core of the bill.

Republican Patrick J. Toomey of Pennsylvania and Democrat Joe Manchin III of West Virginia are zeroing in on a deal to expand the nation’s background check system to require all commercial sellers to keep a record of their sales, according to sources tracking the talks. Manchin told reporters leaving the weekly Democratic caucus luncheon that he would update leaders at 5 p.m., noting that he and Toomey are “still working” on final details. Toomey’s support would be a boon for the legislation’s prospects because he might bring other Republican votes that might provide political cover to colleagues and the House GOP should the Senate approve the measure.

But with or without a bipartisan deal, it is not yet clear whether Democrats will have the votes to clear the 60-vote hurdle to open debate on the bill. Democratic leadership aides are cautiously optimistic that they have the votes but concede that they will need more than just five Republicans to join them. Although the Senate Democratic caucus numbers 55, leaders anticipate losing a few of their own members on the vote.

Manchin, who is on the more conservative end of his caucus’ spectrum, said Tuesday he was confused by the GOP’s filibuster threats.

“I agree with Republicans: There should be an open amendment process, and that’s been guaranteed,” Manchin said, evoking his predecessor, the late Robert C. Byrd, and his love of Senate procedure. “Why would be they arguing and saying that they’re going to filibuster when [they’ll get] everything they’ve asked for, which is an open amendment process? We’re all big guys and big ladies, and we can take the votes and it is what it is.”

But Republicans aren’t the only ones who are mulling over votes against opening debate. Red-state Democrats who are up for re-election in 2014, already wary of gun legislation in general, were noncommittal with regard to supporting debate. Sen. Mark Pryor, D-Ark., is one member that leadership could lose.

Asked whether he would support a filibuster of the Democratic gun bill proposal, Pryor said, “I want to see how this develops.” He added,“I know that many senators are working to try to get some sort of compromise.” Pryor said he opposed the Democrats’ current proposal.

Other Democratic senators who could defect are Mark Begich of Alaska and Max Baucus of Montana.

Manchin noted that he had talked recently with Sen. Charles E. Schumer of New York, who also has been involved in the bipartisan talks. However, aides said that the Democratic messaging chief had been leaving the haggling to Manchin and Toomey, while Schumer tended to the concerns of outside groups.