A threatened Democratic filibuster of a gun rights amendment to the Defense authorization bill has once again highlighted the deep divide between Democratic Senators who favor stiff gun controls and those who back more lenient regulations.

At issue is an amendment offered by Republican Policy Committee Chairman John Thune (S.D.) that would allow gun owners to carry concealed firearms into states with similar gun regulations.

Democratic leaders are working to rally at least 41 Senators to vote against the amendment. But they lack the support of many in the moderate wing of the Conference — most notably Sen. Mark Begich (Alaska), who is co-sponsor of the amendment — and are already two votes down thanks to the absence of ailing Sens. Robert Byrd (W.Va.) and Edward Kennedy (Mass.).

Sen. Claire McCaskill (D-Mo.) said Monday that she would vote against the Thune amendment, noting that gun laws should remain a state issue. Liberal Senators hope McCaskill’s opposition is an indicator of how other independent-minded Members, such as Sen. Jon Tester (D-Mont.), might vote.

But Begich said Monday that he would reach out to his Democratic colleagues to rally support, and Republicans argued that leadership may find it difficult to convince Tester and other pro-gun-rights Democrats to vote “no.—

A senior GOP aide pointed out that Tester and other Democrats in the 2006 and 2008 classes — which gave Democrats control of the Senate — largely ran on platforms supporting gun rights. “It’s one of those issues where during the ’06 and ’08 elections, the Democrats that were elected were talking a very different game than the Washington Democrats,— the aide said, arguing that most of the recently elected Democrats in the Senate are “all fairly conservative on the Second Amendment.—

The growing divide in the Democratic Conference over guns underscores a larger problem for the new majority — keeping unified its more junior moderates and veteran liberals on social issues.

It’s a familiar headache for the GOP — who have seen their power dwindle in the Northeast and other areas as a result of the party’s hard-right tilt during the Bush administration.

“It’s the same problem that Republicans had. … This is how the Northeast Republican seats got into so much trouble, even when they were voting with their constituents,— one Republican Senate aide argued.

Despite the problems supporting a filibuster could cause some of their moderates, however, Senate Democratic leaders on Monday railed against the bill.

“This amendment is a bridge too far, and could endanger the safety of millions of Americans,— Democratic Conference Vice Chairman Charles Schumer (N.Y.) said in a statement. “We will do everything we can to stop this poisonous amendment from being enacted.—

Democrats are closely counting votes for the amendment, which will be considered on the floor as soon as Wednesday. A coalition of 400 city mayors is assisting with the counteroffensive by lobbying Congress, and Democratic Sens. Frank Lautenberg (N.J.), Bob Menendez (N.J.), Dianne Feinstein (Calif.) and Schumer, along with New Jersey Gov. Jon Corzine (D), will speak against the amendment Tuesday.

Democrats were rattled earlier this year when an amendment sponsored by Sen. Tom Coburn (R-Okla.) allowing guns on federal lands usurped legislation granting the District of Columbia a voting Member of the House. Coburn’s amendment has all but killed the bill.