The Senate is poised to either start debate on a Democratic plan to extend some of the Bush tax rates or lapse into more legislative gridlock as both parties angle for political advantage. The vote is set for 2:15 pm on Wednesday afternoon.

For those parliamentary experts out there at home, the Senate is scheduled to vote on the Motion to Invoke Cloture on the Motion to Proceed to the Middle Class Tax Cut Act of 2012, which is bill number S. 3412.

Democrats need 60 votes to force a start to debate on that bill.

It wasn't immediately clear on Wednesday evening if Republicans would also vote to start debate - and then fight over whether they could offer amendments - or if the GOP would just immediately move to scuttle the bill, which would touch off yet another round of finger pointing on the Senate floor.

Democrats seemed to be falling in line behind their leadership, which was fighting to muster a simple majority for the bill.

Already Sen. Joe Lieberman (I-CT) and Sen. Jim Webb (D-VA) had said they would not vote for the plan; but no other Democrats had defected as yet.

That included Sen. Joe Manchin (D-WV), who has run away from most initiatives backed by the White House for months; Manchin said Tuesday he would vote for this bill, which would extend current income tax rates for those making less than $250,000 per year.

Others were acting like they were on the fence, but few believe they are really on the fence, like Sen. Bill Nelson of Florida, who has repeatedly said he wants to vote for a limit of $1 million a year.

But in a quick interview by the Senate subway on Tuesday, I reminded Nelson that the bill on the Senate floor was not his $1 million income limit - instead it was the $250,000 level.

"Are you going to vote for that?" I asked.

"Let's see what's coming up," Nelson said. "I don't know."

Despite Nelson's response, conventional wisdom in the hallways was that the Florida Senator would stick with his Democratic leadership; but we'll see how the votes come down this afternoon.

None of the 47 Republicans are expected to break ranks on this procedural vote.

Their own GOP plan for a one year full extension of all income tax rates will come up for a vote in the House next week.