The Senate on Monday rejected a Democratic proposal intended to ensure millionaires pay a minimum tax rate.

In a mostly party-line 51-45 vote, Democrats fell short of the 60 votes they needed to move forward with a debate on the so-called “Buffett Rule” legislation, with all but one Republican, Sen. Susan Collins (Maine), voting to oppose moving forward on the legislation.

Sen. Mark Pryor (Ark.) was the only Democrat to vote against the motion, though Sen. Joe Lieberman (I-Conn.), who caucuses with Senate Democrats but missed Monday’s vote, released a statement announcing his opposition to the Buffett Rule.