One of the first maxims of the congressional whips is, “If you’ve got the votes, then vote.” Additional delay can only work to unravel a thinly woven majority. So a decision to call the Senate roll on schedule Tuesday on a bipartisan compromise for expanding background checks will be a clear sign that proponents have rounded up the 60 senators needed to guarantee victory.

A decision to put off the vote for one day would mean the plan’s authors don’t have the magic number in hand but are pretty confident of getting there. A delay lasting any longer would spell big trouble for the background check language. And without that amendment, the underlying gun control bill is doomed — meaning the outcomes on the other amendments (most of which would weaken gun control) don’t matter much.

“I think it’s an open question as to whether or not we have the votes. I think it’s going to be close,” Republican Patrick J. Toomey, who wrote the compromise with Democrat Joe Manchin III, said Sunday on CNN.

They were hoping that some of the fence-sitters — the outcome rests with three Democrats and six Republicans — would be won over by the nation’s No. 2 gun rights group, the Citizens Committee for the Right to Keep and Bear Arms, which endorsed the Manchin-Toomey plan on Sunday. The group said it’s an appropriate scale-back from the background check proposal originally pushed by President Barack Obama.

Deal supporters also are hoping that former Rep. Gabrielle Giffords, who plans to plant herself Tuesday outside a main entrance to the Senate floor, will be able to win over a couple of the undecideds by her very presence. They are hoping that a coincidence of history — Tuesday is the sixth anniversary of the shooting deaths of 32 students at Virginia Tech — might cause one of the waverers to fall into their camp.

Finally, they are hoping that one of the Senate’s most vigorous supporters of tougher gun control, the ailing 89-year-old Frank R. Lautenberg, who hasn’t cast a floor vote in six weeks, might be well enough to get to the chamber.

Without Lautenberg, the amendment has 52 solid “yes” votes, or eight short of what it needs.

The New Jersey senator is one of 49 Democrats in favor. Of the remaining six, “no” votes are essentially guaranteed from Heidi Heitkamp of North Dakota, Mark Begich of Alaska and Mark Pryor of Arkansas. That leaves the three others from the “red state five” — senators (including Begich and Pryor) running for re-election in 2014 in states carried by Mitt Romney — as the only other Democrats the National Rifle Association has much of a chance of winning over: Max Baucus of Montana, Kay Hagan of North Carolina and Mary L. Landrieu of Louisiana.

Among the Republicans, only four have so far committed to voting for the background check compromise: Toomey, Mark S. Kirk of Illinois, Susan Collins of Maine and John McCain of Arizona.

And the rest of the lobbying attention is being focused on six of the GOP senators who voted last week to bring the bill to the floor in the first place. Johnny Isakson and Saxby Chambliss of Georgia, Jeff Flake of Arizona, Dean Heller of Nevada, Kelly Ayotte of New Hampshire and John Hoeven of North Dakota.

Chambliss is retiring and none of the others are on the ballot next year.