Washington — Few times in the Senate's recent history has the outcome of a bill been so up in the air as the proposed gun control legislation this week.
The number of senators who may vote for one measure or another is unknown. The shape of a final bill is unknown. And as of Monday, the schedule for debate and votes is also unknown.
"No one knows," said Debra Reed, spokeswoman for the Brady Campaign Against Gun Violence. "But we certainly hope we will have a piece of legislation we can support."
Although gun control advocates wanted a ban on assault-type weapons and high-capacity magazines, they are willing to settle for a bill that expands FBI background checks on gun buyers and cracks down on gun trafficking and "straw purchases."
But there's no guarantee that even those moderate measures will have enough support in the Senate. There's even less chance that a ban on assault weapons and high-capacity magazines would be approved.
Mayors Against Illegal Guns ran ads targeting 13 key senators over the weekend. The organization's spokeswoman, Erika Soto Lamb, said it's unclear where those senators will stand.
"We are all sort of watching different whip counts," she said.
Sixteen Republicans joined nearly all Senate Democrats last week in a vote that moved the gun control bill to the floor. But not all Republican who voted to open debate on the bill support the new restrictions that gun control advocates hope to see in a final bill.
And as many as four Democrats from rural states who face tough re-election next year could join the two Democrats who voted against letting the gun bill go forward. There are 45 Republicans and 53 Democrats in the Senate and two independents who usually vote with them.
On Monday, the liberal group MoveOn.org and Lori Haas, the mother of a survivor of the Virginia Tech shooting, presented a petition signed by 1.3 million people asking for tougher gun laws to those six Democrats — Sens. Mark Begich of Alaska; Mark Pryor of Arkansas; Mary Landrieu of Louisiana; Max Baucus of Montana; Kay Hagan of North Carolina; and Mark Warner of Virginia.
"After a gunman shot and killed 26 children and faculty members at Sandy Hook Elementary School, politician after politician promised this time would be different and we'd actually pass meaningful legislation to help stop these senseless acts," Haas said. "But now all that's in danger of slipping away — because a group of Senate Democrats are possibly wavering in support of common sense measures like universal background checks and a ban on assault weapons and high-capacity magazines."
There's also the possibility the National Rifle Association will use the gun debate to try to weaken current law — or add a "poison pill" to the final legislation that would stop it from winning approval.
One amendment proposed by the NRA and sponsored by its allies in the Senate would force all states to recognize each other's concealed-carry laws. That could override Connecticut's restrictions.
Sen. Chuck Schumer, D-N.Y., a leader in the gun-control debate, said the proposal is dangerous.
"In New York, Times Square is different than rural Wyoming, and our police don't need even law-abiding citizens having the right to carry guns underneath because you can't tell the difference between them and criminals," Schumer said.
Another amendment, sponsored by Sen. Lindsey Graham, R-S.C., is an NRA-backed substitute to the legislation proposed by Democrats.
Graham's amendment would not extend FBI background checks of prospective gun purchasers, now limited to sales at dealers. But it would increase prosecutions of those who fail background checks.
The bill would also include more money for armed school protection and change the requirements for reporting those with mental illness who would fail background checks.
Gun control advocates vigorously oppose Graham's amendment.
"(It) would allow many more individuals who have been involuntarily committed or otherwise found to be seriously mentally ill to buy and possess guns," said Mark Glaze, director of Mayors Against Illegal Guns. "In fact, [the amendment] weakens current law governing these cases."
Senate Majority Leader Harry Reid said the first amendment would be a compromise crafted by Sens. Patrick Toomey, R-Pa., and Joe Manchin, D-W.Va., that would not result in universal background checks, as President Obama wanted, but would expand them to sales at gun shows and on the Internet
"I think the vote is going to be very, very close on the Toomey-Machin amendment," said Sen Chris Murphy, D-Conn.
Murphy also said gun control advocates like himself "are going to be playing both offense and defense."
The Toomey-Manchin legislation got a boost over the weekend when it was endorsed by a gun rights group, the Committee for the Right to Keep and Bear Arms. The committee considers parts of the bill's wins for the gun rights lobby, including the reinforcement of a ban on a federal gun registry and a provision that federal officials would go to prison for up to 15 years if they use gun sales records to set up a registry.
Although the outcome of the debate is unknown, the tenor of it is. It is infused with emotion, especially since a number of gun violence victims and family members of victims, including those shot in Newtown, have lobbied senators intensely and are now a common sight in the halls of the Capitol.
That lobbying will continue and intensify this week. There will be more petitions and visits to senators' offices.
One group hopes to organize an event called "No More Names" that consists of volunteers reading the names of all shooting victims since Adam Lanza killed 20 children and six women at Sandy Hook Elementary School on Dec. 14. The recitation will be on the steps of the Capitol and continue day and night all week long.
Meanwhile, the NRA is intensifying its pressure on congressional allies and has said repeatedly it will monitor all votes this week for "candidate evaluations" sent to nearly five million NRA members each year.
The Senate debate on gun-control legislation is expected to continue into next week.