The Senate's health care reform plan faces its first critical vote Saturday.

Unlike the more disciplined House, where the bill came to the floor and passed on the same day, the Senate process will certainly stretch into December and perhaps beyond.

This weekend's Senate vote is on a "shell bill" — an unrelated House bill that will be entirely replaced with the text of the Senate health care legislation further along in the process.

The vote will be on cloture, meaning it will require 60 votes to begin debate on the bill. Another cloture vote to end debate will eventually be required before the final vote, which only requires 50 to pass.

Still, a new study shows that the Senate eventually passes almost all bills that pass a cloture threshold, so this is an important step in the Democratic push to reform health care.

Here are the undecided Senators whose votes are the most important:

Blanche Lincoln (D-Ark.)

Why she's hedging: Lincoln is up for reelection in 2010 in a very conservative state. Though she's a trusted member of the Democratic Senate leadership, she is facing a potentially hostile political climate in November of 2010.

What's she saying: On Thursday, Lincoln told an Arkansas paper that "health care reform is much needed," adding that Congress should not forget that improving the economy should be a top priority.

What she'll do: Congress.org users give Lincoln a 73 percent chance of voting with the Democratic caucus, and Dick Durbin said that Lincoln has already told Senate Majority Leader Harry Reid (D-Nev.) how she'll vote. Since her vote is so important, it's unlikely that Reid would still be pushing a Saturday vote if she's against cloture. Update: Lincoln said she would support the bill Saturday afternoon.

Ben Nelson (D-Neb.)

Why he's hedging: Nelson has taken issue with the public option included in the bill — even the watered-down compromise public option that allows individual states to opt-out.

What he's saying: On Friday afternoon, Nelson announced he would vote for cloture. "This weekend, I will vote for the motion to proceed to bring that debate onto the Senate floor," he said in a statement.

What he'll do: Congress.org users give Nelson a 69 percent chance of joining with the Democrats, but his public statements suggest that the odds are even higher. Reid has sweetened the deal by removing a part of the bill dealing with insurance companies that Nelson had objected to.

Mary Landrieu (D-La.)

Why she's hedging: Like many of the other hedgers, Landrieu is in a very conservative state. She has also voiced her objection to the watered-down version of the public option in Reid’s bill.

What she's saying: Reid and the Obama administration have been putting a great deal of pressure on Landrieu. CQ reports that, after a November 18th meeting with both Reid and Interior Secretary Ken Salazar, Landrieu's position shifted from "leaning no" to "neutral" — an indication that their lobbying had an effect.

What she'll do: Congress.org users give Landrieu a 77 percent chance of joining the Democrats on the cloture vote. Reid has inserted a provision in the bill granting $100 million in federal money for states that have experienced a natural disaster in the past seven years — and Louisiana certainly qualifies. The perks for her state in the bill may ultimately convince Landrieu to join with the Democrats — at least for the procedural vote. Update: Landrieu announced she would vote for cloture Saturday.

Joe Lieberman (I-Conn.)

Why he's hedging: Lieberman harbors strong reservations about the public option, to the point of openly challenging the Democratic leadership on the issue. Though he's an independent, he caucuses with the Democrats and chairs the Senate Homeland Security Committee.

What he's saying: Lieberman affirmed that he'll join the Democrats in their Saturday vote, but he will reserve judgment on his vote further along the process.

What he'll do: Congress.org users give Lieberman only a 31 percent chance of joining with the Democratic rank-and-file on this first hurdle. His definitive statements to reporters that he will vote yes on the first vote seem to contradict their prediction, however.

Olympia Snowe (R-Maine)

Why she's hedging: Snowe is against the public option, even though she voted to move Max Baucus' version of the bill out of committee. Snowe is one of the last of a dying breed: the New England Republican. She's popular, but she represents a blue state. Recent polling shows she might lose in a Republican primary against a more conservative challenger, but she remains popular among Democrats and the electorate-at-large in her state.

What she's saying: After the Finance vote, Snowe told Roll Call, “My vote today is my vote today. It doesn’t forecast what my vote will be tomorrow.” Recently, Snowe has been mum about the latter.

What she'll do: With all 60 Democrats now seemingly likely to vote to join together on this first bill, Snowe's support is less crucial for Reid. Still, Congress.org users give her a 65 percent chance of joining with the Democrats.

Byron C. Tau writes for Roll Call.