Sen. Ben Nelson (D-Neb.) officially joined Sen. Joe Lieberman (ID-Conn.) as the second member of the Democratic Conference to threaten a filibuster of the Senate’s health care reform bill.

While Lieberman’s objection centers primarily around the measure’s creation of a public insurance option, Nelson said on Thursday that a public option, abortion language and other issues could cause him to vote with Republicans to block the bill from passing at the end of weeks of debate.

Asked if he would filibuster if the bill does not bar federal funding of abortions to his liking, Nelson replied, “Yes. In other words, I won’t vote for cloture on the motion to end debate. But I don’t want to get involved in each and everyone of the details because there are a lot of other things that could keep me from supporting it at the end as well if they aren’t to my satisfaction.—

Up until Thursday, Nelson had held out the possibility that he might support a filibuster, but he had not specifically threatened to do so.

Still, Nelson reiterated that he has not yet decided whether to join the expected GOP-led filibuster attempt of the motion to proceed to the bill. That vote is likely to occur Saturday around 8 p.m., Democratic leaders said. In fact, Nelson appeared to be leaning toward voting to start debate on the bill.

“There will be opportunities to try to change it. There probably will be negotiations it at the end if there aren’t 60 votes, and the question is do you keep it alive for another day or two,— he told reporters. Lieberman has said he will support starting the Senate debate, but will vote to block the final bill if it includes a public option.

Given unanimous Republican opposition to the bill as it is currently written, Majority Leader Harry Reid (D-Nev.) will need the votes of all 60 members of the Democratic Conference to start debate on the bill, and if the Republicans remain united, he will need them all to end debate on the measure. Sixty votes are needed to overcome a filibuster.

Another Democratic holdout, Sen. Mary Landrieu (La.) said Thursday that she’s only gone through about 100 pages of the 2,074 page bill so far and hasn’t decided how she’ll vote on starting debate.

“I am in the neutral column until I finish reading the bill,” she said, adding that she is scrutinizing the affordability for small businesses and individuals, as well as the particulars of the public option, which she said she would like to see narrowed even further. She also doesn’t like having the effective date of the bill pushed back from 2013 to 2014.

But Landrieu said she is in wheeler-dealer mode, rather than having a fundamental objection to the overhaul measure.

“I have my leverage now,” she said. “I’m using it to the best of my ability.”

She said she’s been in meetings with Nelson and Sen. Blanche Lincoln (D-Ark.), and it’s possible the trio will issue a joint statement on how they will vote on the motion to proceed.

“We have similar views,” Landrieu said.