UPDATE: As expected, the Manchin-Toomey compromise amendment failed to clear a GOP-led filibuster this afternoon. In the end, 54 senators voted in favor of it, six shy of the needed 60 votes.
Those Republicans crossing party lines to back the measure: Pat Toomey of Pennsylvania, Susan Collins of Maine, Mark Kirk of Illinois, and John McCain of Arizona. Those Democrats who sided with Republicans to block it were: Max Baucus of Montana, Mark Begich of Alaska, Heidi Heitkamp of North Dakota, and Mark Pryor of Arkansas. (Senate Majority Leader Harry Reid switched his vote to a no at the last minute, but that was a procedural move to allow him to bring the measure up for again later if he wants.)
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Original Post 11:23 a.m.: A nominally bipartisan effort to save the Senate's gun-control bill appears likely to fail this afternoon, delivering a major blow to gun-safety advocates and casting serious doubt as to whether Congress will enact significant gun reform in the wake of Newtown.
The upper chamber will begin voting on a series of nine amendments to an existing bill late this afternoon. Up first is the most important: a compromise amendment from West Virginia Democrat Joe Manchin and Pennsylvania Republican Pat Toomey that would expand background checks to cover Internet and gun show sales. That measure is crucial to saving the bill because it replaces a more expansive background-check provision currently in the bill that is as a non-starter with conservatives.
The problem for Manchin and Toomey is that their compromise has proved only slightly more attractive to the pro-gun rights crowd. The pair has so far only managed to convince two other Republicans—Susan Collins of Maine and Mark Kirk of Illinois—to support the effort. Of the remaining 42 GOP senators, all but John McCain has made it clear to varying degrees that they will vote no when the time comes. Given Senate math, that makes it impossible for even a united Democratic caucus to find the 60 votes they'd need. (The Huffington Post has a handy whip count here; Manchin's also grudgingly admitted that he doesn't currently have the votes.)
After the Manchin-Toomey vote, the chamber is also set to consider a handful of more partisan amendments. Democrats are offering measures that would reinstate an assault-weapons ban, and impose a ban on high-capacity ammunition magazines. Republicans, meanwhile, are offering a proposal that would mandate that any state concealed-carry permit would be honored virtually by every other state, among other proposals.
Votes are set to begin at 4 p.m.