Bob Corker has long been one of Senators thought to be gettable on gun background checks. He is a relative moderate, doesn’t have the posture of an ideological bomb thrower, and has been known to be willing to work with Democrats on occasion.

But Corker will oppose the Manchin-Toomey background check compromise. His spokesperson, Laura Herzog, emails:

“Senator Corker would not support Toomey-Manchin as written but is open to supporting amendments to achieve what he believes is the central issue: preventing violence by dangerous, mentally ill people.”

Senator Lindsey Graham’s office also announced today that he will oppose the proposal, as did Senator Tom Coburn. That’s too bad, since Coburn has endorsed the policy goal of expanding background checks, only purporting to have concerns about record keeping.

That’s all bad news, but there are a few bright spots.  Senator Kay Hagan, a red state Dem who was previously undecided, will now support the Manchin-Toomey compromise. Meanwhile, the office of Senator Dean Heller — another potentially gettable Republican — tells me he’s still reviewing the legislation. And ignore all those reports claiming that Senator Heidi Heitkamp is a No; her office confirms to me this morning that she’s still undecided.

So the key Senators to watch now are: Heller, Kelly Ayotte, and Richard Burr on the Republican side, and Heitkamp and Joe Donnelly on the Democratic side. Right now, three Republicans (Kirk, McCain and Pat Toomey) support the proposal. Dems probably need around four more Republicans to pass this.

Another thing to watch: Will red state Dems really vote against the proposal, if it emerges that it could pass with their support? In other words, if Dems pick up a handful of Republicans, and a vote against the proposal from Mark Begich, Mark Pryor, Heitkamp or Donnelly would sink it, the pressure will be extremely intense on them to support it. (By contrast, if it is either going to pass or fail without them, Harry Reid might give them room to vote No.)

I noted this morning that this vote is a major test for self described moderate Republicans who still claim to be willing or able to compromise with Dems in at least some areas to solve the country’s problems. Corker is shaping up to be a disappointment. McCain and Collins (and of course Pat Toomey) are doing the right thing, and Heller and Burr still may yet. It will be interesting to see if red state Dems will prove willing to kill a proposal even though they have the cover offered them in the form of support from Pat Toomey, John McCain, and Joe Manchin.

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UPDATE: One other question for Senators who say they’re a No: Will they at least vote to end debate on the proposal, which would still allow them to vote against the final proposal during the simple majority vote? Or are they saying they’ll vote against cloture to end debate?

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.