Correction Appended

Eight Senate Republicans broke with their leadership Tuesday morning and voted against a mandatory ban on earmarks, while seven Democrats switched sides to vote for the ban as the Senate defeated the proposal by a 56-39 vote.

Sen. Tom Coburn offered the ban as an amendment to a food safety bill that has been held up for weeks as he and Majority Leader Harry Reid (D-Nev.) worked out an agreement on a host of amendments the Oklahoma Republican had sought.

Republican Sens. Bob Bennett (Utah), Thad Cochran (Miss.), Susan Collins (Maine), James Inhofe (Okla.), Dick Lugar (Ind.), Lisa Murkowski (Alaska), Richard Shelby (Ala.) and George Voinovich (Ohio) voted against the ban, despite the fact that earlier this month their Conference adopted a similar — although voluntary — ban that applies only to Republicans.

On the Democratic side, Sens. Evan Bayh (Ind.), Michael Bennet (Colo.), Russ Feingold (Wis.), Claire McCaskill (Mo.), Bill Nelson (Fla.), Mark Udall (Colo.) and Mark Warner (Va.) all voted in favor of banning earmarks.

Although the ban drew more than expected Democratic support, the level of GOP opposition was of particular note given the GOP’s seeming unity just two weeks ago in swearing off the practice of earmarking.

During a closed-door meeting earlier this month when the ban was adopted, only Inhofe spoke against the proposal despite misgivings among many of the party’s old guard.

Lugar’s opposition to the ban is also significant. Tea party activists have become increasingly hostile to the veteran lawmaker over his close relationship with President Barack Obama, his decision to not join in a legal challenge to the health care reform law and his support of the DREAM Act immigration bill.

Tea party groups have made earmarking something of a litmus test for whether they will support an incumbent or look to field a conservative challenger during their primary.

With the defeat of the mandatory ban, it is unclear whether Republicans will stick to their voluntary pledge. Inhofe has already said he will ignore the GOP-only ban, and other opponents have indicated they will likely follow suit.

But even some Republicans who have supported both bans have already broken their pledge. Before the Thanksgiving recess, Minority Whip Jon Kyl (R-Ariz.) added a provision for an Indian tribe in his state into a bill designed to settle legal complaints against the Department of Agriculture by black farmers.

Correction: Nov. 30, 2010

An earlier version of the story contained incorrect numbers for Republican votes.