Advocates are frantically lobbying a small and rapidly shrinking list of undecided senators from both parties who control the fate of the most ambitious gun control legislation to reach the floor in nearly 20 years.

Senate leaders spent Monday quietly working to set up a vote later this week on the proposal, which would require background checks for all private firearms sales over the Internet and at gun shows. Current law requires background checks only for commercial gun sales made through licensed dealers.

In the meantime, the spotlight swung to the handful of senators who are still evaluating whether to vote for or against the measure, cosponsored by West Virginia Democrat Joe Manchin III and Pennsylvania Republican Patrick J. Toomey. The measure will be voted on as the first amendment to broader legislation (S 649) that also would create new criminal penalties for gun traffickers and authorize $40 million a year for school security improvements.

Undecided Democrats included Max Baucus of Montana, Joe Donnelly of Indiana, Heidi Heitkamp of North Dakota and Mary L. Landrieu of Louisiana. Undecided Republicans included Kelly Ayotte of New Hampshire, Saxby Chambliss of Georgia, Dean Heller of Nevada, and Jeff Flake and John McCain of Arizona. The two Arizonans appeared to be headed in different directions, as Flake said he is inclined to vote against the measure and McCain said he is “favorably disposed” to supporting it.

Every senator’s vote will be critical as gun control supporters hope to push the proposal over the 60-vote threshold likely to be required. The measure appeared to have the public support Monday of at least 52 senators, including 49 Democrats and independents and three Republicans: Toomey, Susan Collins of Maine and Mark S. Kirk of Illinois.

One Democrat who had been a holdout, Jon Tester of Montana, said Monday night he had read the amendment and would support it.

But the task of finding an additional eight senators to support the plan appeared to grow more daunting for gun control supporters Monday. Several previously undecided Republicans — including Bob Corker of Tennessee, Lindsey Graham of South Carolina, John Hoeven of North Dakota and Johnny Isakson of Georgia — announced Monday that they either oppose the amendment or are very likely to oppose it.

Senate Majority Leader Harry Reid, D-Nev., signaled that Democrats are still rounding up votes as he declined to file cloture on the amendment and instead said he was negotiating with Republicans to come to a mutual agreement. Democratic leaders initially hoped to vote on the Manchin-Toomey plan Tuesday.

“I continue to work toward an agreement to vote on this compromise,” Reid said. At the same time, he warned Republicans against filibustering the amendment, saying that would be a “shameful tribute” to those who were killed in the shooting at Sandy Hook Elementary School in December.

Graham Pushes Alternative

Graham’s opposition to the compromise, in particular, could spell bad news for gun control supporters since he intends to offer as an amendment competing legislation (S 480) that would change the background check system, but not expand it. The proposal aims to share more records of mentally ill individuals with a national database of prohibited gun purchasers.

The bill is co-sponsored by Flake and Heller, and the trio could be inclined to band together and push their own proposal rather than support the Manchin-Toomey measure. In addition, two Democrats facing tough re-election fights next year — Mark Begich of Alaska and Mark Pryor of Arkansas — also are co-sponsors of the mental health records legislation and are seen as likely to vote against the Manchin-Toomey measure.

“Instead of expanding a broken background check system, let’s fix it,” Graham said in a statement Monday. “I have bipartisan legislation which would accomplish that goal. I hope the Senate will soon debate and approve my legislation.”

But adoption of Graham’s measure at the expense of the Manchin-Toomey bill would amount to a major defeat for at least one gun control group, Mayors Against Illegal Guns, which said Graham’s proposal would be counterproductive and would result in fewer, not more, mental health records being shared with the prohibited buyer database.

Pressure Increases

Despite signs of increasingly long odds, gun control groups expressed confidence Monday about their chances and prepared to ramp up their lobbying in what is a crucial week for the Obama administration’s agenda on guns.

Former Rep. Gabrielle Giffords, D-Ariz., was set to make her way to Capitol Hill to lobby personally for the amendment. Survivors of mass shootings were planning to use Tuesday — the six-year anniversary of the massacre at Virginia Tech — to call for adoption of the Manchin-Toomey amendment.

Mayor Against Illegal Guns, meanwhile, highlighted what it said was an effective advertising campaign against Flake, who voted against expanding background checks in the Senate Judiciary Committee.

“In April, 43 percent of voters disapproved of Senator Flake’s performance as a senator, a net increase of 9 percent from before the advertising began,” the group said in a press release designed to increase pressure on Flake to support the Manchin-Toomey measure.

Other groups said they are not giving up.

“I think there are a number of Democrats and Republicans who want to do the right thing, and the more they hear from people in their states, the more apt they are to vote on our side, and that’s where we’re putting our energy,” said Brian Malte, director of mobilization for the Brady Campaign to Prevent Gun Violence, another gun control group that is pushing for the Manchin-Toomey measure.