Just Toomey, Collins, Kirk, and Maverick John McCain so far, but expect a few more to flip soon. Jeff Flake’s on the fence but he’s probably worried about voting no on gun control across the board when fellow Arizonan Gabby Giffords is campaigning hard against it in the media. (Since McCain’s apparently voting yes, Flake has some cover to follow suit.) Dean Heller has also been galloping towards the center ever since his close call against Shelley Berkley in last year’s Senate race in purplish Nevada. That’s six Republicans — normally enough to break a filibuster when the Senate’s 55 Democrats vote together. Are they voting together this time? Pryor and Begich voted against even considering the bill, so presumably they’re both no’s the rest of the way. Frank Lautenberg’s ill and unlikely to be on the floor this week, so that’s 58 for Reid right now.

Who’ll be numbers 59 and 60?

There are a dozen other Republicans who voted for a motion to proceed on the gun control bill last week, including Sens. Kelly Ayotte (N.H.), Jeff Flake (Ariz.), Saxby Chambliss (Ga.), Johnny Isakson (Ga.) and Dean Heller (Nev.).

Flake on Friday was reviewing the bill while Heller’s office said the senator “will not support any plan that creates a federal gun registry.” Corker is reviewing the bill, according to The Tennessean. Chambliss has made it clear that he opposes the underlying gun control bill that is headed to the Senate floor…

Centrist Democrats who are expected to vote for Manchin-Toomey are Sens. Robert Casey Jr. (Pa.), Kay Hagan (N.C.), Tim Johnson (S.D.), Joe Donnelly (Ind.), Claire McCaskill (Mo.) and Mark Warner (Va.).

But Democrats who declined to comment or didn’t say definitively where they stand on Manchin-Toomey include Sens. Max Baucus (Mont.), Heidi Heitkamp (N.D.) and Mary Landrieu (La.). Baucus and Landrieu are seeking reelection in 2014 and are top GOP targets.

News is breaking on Twitter as I’m writing this that Hagan is a yes. Why would a red-state Democrat from the south commit to a bill on background checks when she’s up for reelection next year? Because: Reid’s going to give her lots of opportunities to vote no on other gun-control amendments to make voters back home happy. The whole reason Feinstein’s assault-weapons bill and a bill on high-capacity magazines will be brought to the floor is so that vulnerable Dems like Hagan and Landrieu and vulnerable blue-state GOPers like Collins can drive a stake through them and claim some pro-gun bona fides in their reelection campaigns even while they’re voting yes on background checks. Think it’ll be enough to placate gun-rights groups? In Collins’s case, at least, I’m thinking no:

The Republican conflict came to the fore last week during a closed-door luncheon for Senate Republicans, when Senator Susan Collins, of Maine, eyes blazing, stood up and complained about a series of attack ads that she was facing back home from a gun-rights group with deep ties to Senator Rand Paul, Republican of Kentucky…

Her comments, according to several Republican aides, ignited a tense debate, similar to many the party has faced since its loss in the race for the White House last year. Senator Rob Portman, Republican of Ohio, stood to say he had been raising money for Ms. Collins’ re-election, only to watch her have to spend it to defend herself against the attack from the gun group, which has been directed at other members as well…

Senator Ted Cruz of Texas, a freshman ally of Mr. Paul’s, jumped in to promise he had nothing to do with the group, according to officials briefed on the event. Then Mr. Paul, feeling attacked, stormed out. (A spokeswoman for Mr. Paul did not respond to repeated requests for comment.)

One other X factor here is immigration. My assumption is that vulnerable Dems are more likely to vote yes on the Gang of Eight bill then they are on the Toomey/Manchin bill simply because they’re likely to have more Republicans voting with them on immigration. But that vote always carries some risk in red states, and if Landrieu and Pryor and the rest are committed to it, then maybe they feel they have no choice but to vote against gun control in its entirety, including Toomey/Manchin, to retain some goodwill with conservatives back home. Hard to gauge, though, when the fate of immigration reform is still so dicey. What if Landrieu’s expecting/hoping that the Gang of Eight bill will collapse soon, now that conservatives are about to turn their full focus to it? In that case she’ll never get a chance to vote, which means she can afford to be a little squishier on background checks to appease liberals. Lots of moving parts in the Senate machinery right now. I wonder who’ll end up getting sucked into the machine and flattened next November.

Here’s Alan Gottlieb of Citizens Committee for the Right to Keep and Bear Arms insisting that Toomey/Manchin is actually a total win for gun-rights supporters and that they should get behind it. The NRA, needless to say, does not agree.

Update: Don’t overlook the possibility of a cloture/final vote switcheroo either. Reid can probably get to 51 votes on the final bill even without red-state Democrats. Where he really needs them is on the cloture vote, to beat a GOP filibuster. How many of them will be willing to vote yes on cloture and no on the final bill on the theory that gun-rights fans in their home states care more about the final vote than the cloture vote?