This is a big deal, and not just because of the swing of a single vote in the Senate:

NBC News has learned Sen. Kelly Ayotte will be a NO on background checks bill today.

— Kelly O'Donnell (@KellyO) April 17, 2013

“Last week, I voted to initiate Senate debate on preventing violence and protecting the Second Amendment rights of law-abiding Americans. Several proposals have been brought forward, and I’ve been carefully reviewing them all.

“There are responsible steps that can be taken to stop criminals and others who are already prohibited from possessing firearms under federal law from obtaining them. With those principles as a guide, I am cosponsoring the Protecting Communities and Preserving the Second Amendment Act, which includes needed reforms to the National Instant Criminal Background Check System, addresses mental health gaps in the criminal justice system, and criminalizes gun trafficking and straw purchases.

“To further address the need to improve the nation’s mental health system, I also am co-sponsoring the Mental Health Awareness and Improvement Act. This bipartisan measure includes provisions of legislation I helped introduce that seek to improve mental health first aid training and increase the effectiveness of mental health care across the nation.

“I believe that restricting the rights of law-abiding gun owners will not prevent a deranged individual or criminal from obtaining and misusing firearms to commit violence. While steps must be taken to improve the existing background check system, I will not support the Manchin-Toomey legislation, which I believe would place unnecessary burdens on law-abiding gun owners and allow for potential overreach by the federal government into private gun sales.”

Presumably, Joe Manchin already knew this when he threw in the towel on his bipartisan compromise with Pat Toomey earlier today, but the announcement all but guarantees the demise of Democratic gun-control proposals scheduled for votes today.  The “defection,” if it can be called that, of a Republican who voted to allow debate on the bill will make it very difficult to convince red-state Democrats like Mark Pryor to support it — especially since it’s clear that it will go down to defeat.  Why risk the ire of 2014 voters just to back a failure?

If you missed it last night, go now and read Allahpundit’s explanation of Ayotte’s influence on the Senate Republican caucus, and how Ayotte signaled this from the beginning.  It now puts Democrats on defense, because the GOP didn’t just sit back and say, “No.”  They have an amendment of their own to counter Manchin-Toomey that addresses mental-health reporting in the existing background-check system, as well as authorizing better enforcement of it.

With the bottom falling out from under the entirely non-sequitur attempt to link background-check gaps to Newtown, in which background checks actually stopped the shooter from buying weapons legally, will Democrats support a solution that actually addresses some of the issues of the incident they’ve been citing as their inspiration?  Or will they kill the GOP amendment and leave themselves open to the same accusation they intended to level against Republicans in the event of failure of the bill?  If the Senate passes it, don’t expect Barack Obama to veto it and put himself in that position.

Update: Talk about strange timing:

Senate Majority Leader Harry Reid announced support for an assault weapons ban Wednesday, a change in his position.

“I’ll vote for the ban because saving the lives of police officers, young and old, and innocent civilians, young and old, is more important than preventing imagined tyranny,” the Nevada Democrat said on the Senate floor.

The statement marked a reversal for Reid, who voted against renewing the assault weapons ban in 2004. Earlier this year, Reid opted against bringing an assault weapons ban penned by Democratic Sen. Dianne Feinstein to the floor.

Reid said he had had to “reassess” his position after listening to the arguments against an assault weapon ban and deciding that they were “absurd.”

Not as absurd as the term “assault weapons.”  Putting that aside, why flip now? Why take the political hit when the amendment is going to fail, and fail badly, within a couple of hours?  The time to support this amendment was a few months ago, when Reid’s influence might have helped with his red-state Democratic colleagues.  I doubt this will win Reid any new friends in Nevada, and it will probably cost him a lot of support … if he decides to run for another term.

Update: This is a body blow for Democrats:

Terrible. @heidi4ND will vote against Toomey-Manchin today, a statement from her office confirms.

— Greg Sargent (@ThePlumLineGS) April 17, 2013

With Heitkamp out, maybe Reid will think twice about pursuing the bill at all.

Update: An amusing point from Dave Weigel:

Losing Heitkamp, who's not up for re-election until the iPad 9 is released, is a big fail for Manchin-Toomey

— Dave Weigel (@daveweigel) April 17, 2013

Did anyone bother to check with her first?