Sen. Ben Nelson is emerging as the Senate Democrats’ hardest sell on health care reform, and the defeat of his anti-abortion amendment Tuesday could prove to be the tipping point for the Nebraska centrist.

“It makes it harder to be supportive— of the health care bill, Nelson said after his amendment was defeated. He added, “We’ll have to see if they can make it easier.—

Nelson failed to muster even a bare majority for his proposal to expand restrictions on federal funding of abortion. The amendment was killed on a 54-45 vote.

Nelson has said he may be inclined to filibuster any bill that does not prohibit insurance companies from offering abortion coverage to people who receive federal health insurance subsidies, as his amendment would have done.

In their quest to secure a filibuster-proof 60 votes for the overhaul, Democrats said Nelson’s support was appearing unattainable — with or without the inclusion of his abortion provisions — given the myriad objections he has to other parts of the bill, including the creation of a public insurance option.

“The concern about Sen. Nelson is that he’s gotten himself pretty far out on a couple of limbs,— said one senior Senate Democratic aide. “One of them is abortion, and I don’t see how he finds the middle ground on an issue that has eluded people for 40 years. The other is on the public option, and he seems to be increasingly boxed into a corner if he can’t accept— a new proposal being negotiated by a group of 10 Democratic moderates and liberals.

Though Majority Leader Harry Reid (D-Nev.) secured the votes of all 60 members of the Democratic Conference to begin debate on his $848 billion bill, he has been struggling to persuade the same 60 to agree to support an end to that debate. Because of the very real prospect of losing Nelson’s vote, as well as the vote of self-described Independent Democrat Joe Lieberman (Conn.), Democratic leaders and the White House have intensified their efforts to make sure the measure appeals to Maine’s GOP centrists Olympia Snowe and Susan Collins. Collins met with President Barack Obama on Monday, while Snowe visited him on Saturday. Both have been fielding calls from Senate Democrats anxious to assuage their concerns.

Majority Whip Dick Durbin (D-Ill.) declined to say Tuesday whether Nelson, Lieberman, Snowe or Collins would be the hardest vote to snag.

“I don’t think I can grade them. I can just tell you we need them all,— Durbin said. “The effort’s being made for all of them, and there’s still an outreach effort on the other side of the aisle.—

On the surface, it would appear to be good news for Nelson that a group of five moderates and five liberals — of which he is a participant — has been trying to craft a deal that would do away with the bill’s public option. But Nelson has appeared cool to the potential deal, which Members hoped to finalize after press time Tuesday evening.

Following the group’s Monday evening huddle, Nelson said he was indeed apprehensive about signing on to the prospective agreement.

“If I seem reluctant, it’s because I think others are beginning to see how difficult it is to put together a program on the basis on what you want for government versus what it would require in terms of the technical nature of actuarial science,— he said. “At the end of the day, the numbers have to work or somebody is left holding the bag.—

Nelson said Tuesday that he could not even say how he is leaning at this time, given the uncertainties.

“Until everything is settled and you’ve got a clear view of what changes might be made and what may be there, it’s hard to say,— Nelson said when asked whether he felt he could be supportive of the final health care plan. “You can’t quantify in terms of percentages, whether I’m halfway there or things like that, because I’m continuing to try to work with the variety of issues with my colleagues to try to be at least a friend of the process.—

But he acknowledged that losing his abortion amendment “could— be the determinative factor.

“I don’t want to be stubborn and closed-minded [but] I have trouble imagining what [another compromise] would be, but there are other people with great imaginations and perhaps they’ll come up with something, but my goal is to get the Nelson-Hatch amendment passed,— Nelson said before the vote on his proposal.

Reid said Tuesday that he hopes Nelson would continue working on an abortion compromise.

“If in fact he doesn’t succeed, we’ll try something else,— Reid said.

But Democrats privately said they do not think Nelson will end up voting for the health care overhaul, and the likelihood of losing both Nelson’s and Lieberman’s votes is very real.

“I think we lose one of the two of them for sure,— said the senior Senate Democratic aide. “If I’m reading it right, we lose Nelson for sure, and there’s a greater than 50 percent chance we lose Lieberman, too.—