WHAT DRIVES BAYH?…. Just last week, Sen. Evan Bayh (D-Ind.) explained that he has no intention of “watering down” President Obama’s agenda; he actually wants to “strengthen and sustain it.” The goal, Bayh said, is make Democratic proposals more palatable to Senate Republicans, so that the GOP’s filibusters will be easier to break.

And yet, last night, the Senate voted on the budget, and there was no threat of a GOP filibuster. The majority could simply pass the plan they wanted to pass. Every Senate Republican voted against it, as did the Democratic caucus’ most conservative member, Sen. Ben Nelson of Nebraska. Evan Bayh, the one who has vowed to “strengthen and sustain” the president’s agenda, joined with Nelson and the Republicans.

Nelson’s vote wasn’t too big a surprise; he represents a pretty conservative “red” state. Bayh’s motivation is a little tougher to understand. As Matt Yglesias noted this morning:

I’ve heard some see this as an act of political cowardice on Bayh’s part, but I think that’s wrong. Obama carried Indiana. There are many Senate Democrats in more vulnerable states who voted “yes.” Bayh just made a decision of conscience and principle to stand with Mitch McConnell and Jim DeMint on the most important domestic policy vote of his career.

I think that’s right, and I’d add one key detail. Just yesterday, Sen. John Cornyn (R-Texas), the chairman of the National Republican Senatorial Committee, effectively conceded that Republicans won’t go after Bayh next year. “We have a sort of priority list,” Cornyn said. “He’s down on that.”

Bayh, in other words, represents a state that supported Obama in November and he knew that no matter how he voted of Obama’s budget, his re-election is all but assured in his home state.

Bayh didn’t have to worry about impressing conservative voters back home; he didn’t have to worry about fundraising; he didn’t have to worry about a Republican opponent back home using this budget vote against him. Bayh was free to vote however he pleased.

And given all of this, Bayh still sided with a right-wing Republican caucus against the Obama White House.

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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.