UPDATE: The brand-new Republican replacement bill I mention below is online here.
Sam Stein and Amanda Terkel have been keeping up a solid whip count on Manchin-Toomey. The gun bill on the floor of the Senate right now is the un-watered-down Schumer version; if Democrats got their way, the Manchin-Toomey compromise would replace all that, and 60 senators would vote for cloture to move ahead to a full vote. And Manchin-Toomey would win on a full vote. Fifty-two senators support it.
Until today, six Democrats were undecided on Manchin-Toomey, as were two Republicans. If every one of those senators voted for cloture today, the eventual bill would succeed. But comes now NBC News' Kelly O'Donnell:
Ayotte will support an alternative gun bill offered by @chuckgrassley . Manchin-Toomey backers had hoped to get Ayotte but won't.— Kelly O'Donnell (@KellyO) April 17, 2013
That's the ballgame. If every on-the-record senator means "I'll oppose cloture" when he says "I won't vote for it," that means there's at most 59 votes for cloture. That means wavering Democrats won't be confident that they can cast a tough vote and it'll mean anything beyond attack ads.
Meanwhile, 20-odd Republicans have regrouped and endorsed a last-minute replacement bill, the text of which Republicans have not yet handed over to Democrats. They do have a helpful fact sheet.