With Ben Nelson now in the "yes" column, there are now two known Democratic hold outs on tomorrow's health care vote: Sen. Mary Landrieu (D-LA) and Sen. Blanche Lincoln (D-AR).

Landrieu told reporters today that she'll likely make an announcement tomorrow morning. Lincoln, on the other hand, has been unreachable, and it's unclear if, or when, she'll announce her intent publicly before the vote, which will come at 8 p.m. tomorrow night.

It's probably a safe guess that, if at the end of the day, there's something standing between Senate Majority Leader Harry Reid and 60 votes on the motion to debate to his bill, he won't hold the vote. As unlikely as that is, here are the potential hangups.

• A surprise "No" from Landrieu or Lincoln: Sen. Joe Lieberman (I-CT) is threatening to filibuster the bill at a later date, but he's indicated that he'll vote "yes" tomorrow. Landrieu and Lincoln, on the other hand, are biding their time.
• An unexpected absence: Sen. Max Baucus (D-MT) has been in Montana for a couple days, attending to his mother, who is reportedly very ill. Separately, Sen. Robert Byrd (D-WV) has been in bad health for quite a while, and, though he's been there when he's been needed, he has also missed a number of votes this year. If either, or both men can't make it to the Capitol tomorrow, it will likely cause a (potentially very long) delay.

Those are the updates we'll be bringing you tomorrow, along, of course, with the breakdown of the vote itself. If the clerk calls the role, the outcome won't be in doubt--but we may not know until quite late.