To the surprise of absolutely no one, Senate Republicans blocked consideration of the Buffett rule last night with a filibuster. The bill would have set a millionaire’s minimum effective tax rate at 30%, with a phase-in between $1 million and $2 million and an exemption for charitable donations. But Senate Republicans wouldn’t let it get onto the floor for debate.

Today, the Senate will vote on a motion to proceed to the Buffett rule, formally known as the Paying a Fair Share Act. A filibuster is expected, about 5:30 p.m. ET. This concludes a two-week push by Senate Democratic leaders and the Obama Administration on the bill, and the larger conversation around tax fairness and the need for the rich to pay a higher amount than those in the middle class.

As they search for a attractive agenda, the Obama campaign has latched on to the Buffett rule, the alternative minimum tax for millionaires, as a key campaign plank. The fact that they will face Mitt Romney in November, someone who pays less than a 15% effective tax rate, doesn’t hurt either.

This millionaire’s minimum tax also seems like a clever way to eliminate the “carried interest” loophole, where income from money managers is taxed at the capital gains rate of 15% rather than the top marginal income rate of 35%.