This vote was on whether to call for a revote on an amendment that was previously defeated. 

The amendment in question, by Arlen Specter, R-Pa., would have modified the budget resolution to allow for a reduction in the Alternative Minimum Tax (AMT), effectively rolling it back to pre-1993 levels.  This would have the effect of exempting some 21 million Americans who would otherwise be subject to the AMT.  The amendment would not make up for the loss of tax revenue by raising revenues in other ways.  Democrats had objected to the amendment because it would increase the deficit.  This amendment was offered to the fiscal 2009 budget resolution, which sets overall spending targets for the Appropriations committees and outlines other budget rules.

The amendment had earlier been defeated by one vote, but Robert Menendez, D-N.J., made a motion that the vote be reconsidered on behalf of John Cornyn, R-Texas, who had missed the earlier vote.  Democrats tried to block the revote request with a parliamentary maneuver, but were unsuccessful.  This vote was on whether or not to actually take another vote on the amendment. 

The vote was tied, 50-50.  A tiebreaking vote was then cast by Vice President Dick Cheney in his role as president pro tempore of the Senate.  All but one Democrat present voted against the motion to take a revote (Evan Bayh of Indiana).  Every Republican voted for the motion to take a revote.  The end result was that the motion carried 51-50, and a revote was called on an amendment to scale back the AMT without measures to avoid increasing the deficit.