SENATE BAN ON EARMARKS FALLS FAR SHORT…. Some conservative Republicans were already able to convince the Senate GOP caucus to support a self-imposed moratorium on earmarks, but the intra-party measure is non-binding and doesn’t carry the force of law. This morning, they took the next step, pushing a proposal to ban earmarks altogether.

The Senate considered a similar measure in March, and it failed with only 29 votes. This morning, the proposal, championed by Sen. Tom Coburn (R-Okla.) garnered far more support, but still fell far short, 39 to 56.

The vote did not fall along party lines. Eight Republicans — Bennett, Cochran, Collins, Inhofe, Murkowski, Shelby, Lugar, and Voinovich — broke ranks and opposed the measure. A closer look at this list reveals that two of the eight are retiring from the chamber, while the other six are members of the Appropriations Committee, which just happens to be responsible for handing out earmarks. Meanwhile, seven Democrats — Bayh, Feingold, McCaskill, Bennet, Bill Nelson, Udall, and Warner — voted for Coburn’s measure.

But for the real entertainment, take a look at who voted against the earmark ban in March, only to turn around and vote for it this morning. Dave Weigel flags the highest profile example.

In March, an earmark moratorium went down 68-29, and [Maine Sen. Olympia Snowe] voted against it. Today the moratorium failed by a 56-39 vote, and Snowe was among the Republicans who switched her vote to [support a moratorium].

What changed? Bob Bennett, Mike Castle, and Lisa Murkowski lost Republican primaries. Tea Partiers have made it crystal clear that they’re going to challenge Snowe in 2012, with a resurgent Republican electorate in that state clearly ready for the fight. This, again, is the real impact of the Tea Party movement. Whether it costs Republicans a seat or two is almost irrelevant. Its ability to force discipline and demand ideological concessions from Republicans is uncanny.

The same, by the way, can be said of Sen. Kay Bailey Hutchison (R) of Texas, who also switched positions, and who appears likely to face a primary challenger in 2012.

Tea Party zealots may be lacking in a lot of areas — no clear agenda, no leadership, no internal structure, and no real areas of expertise — but they’ve successfully scared the hell out of plenty of GOP lawmakers.

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Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.