Even though his forthcoming proposal has outraged some liberal Democrats such as McCaskill, Nelson has threatened to reject the final bill if it is not included, putting an all-important 60-vote count in jeopardy.
"At the end of the day we need Senator Nelson's vote. We still don't have a promise of a vote from the Republican side, so we would need his vote," Sen. Dick Durbin, D-Ill., told reporters Friday.
The issue of abortion in the Senate has sparked nationwide debate. Anti-abortion groups say they will vigorously oppose the legislation if the abortion amendment does not pass.
"The key thing is that without an amendment that explicitly bans fed funding for abortion, you're going to see the largest expansion of abortion since Roe v. Wade," said Charmaine Yoest, president of Americans United for Life, a public-policy organization that opposes abortion on demand. "We see this bill as abortion business bailout. We don't want see the American taxpayers money going to subsidize abortion in this country."
Groups that favor abortion rights, on the other hand, say any extension of restriction beyond what is in the current Senate bill will move the federal government into a whole new area of restricting women in the private sector from getting abortion services, even if they pay for the premiums.
"Our position is that where the Senate bill started was already a compromise for pro-choice supporters in this country," said Nancy Northup, president of the Center for Reproductive Rights. "And that compromise should be satisfactory to any anti-choice senators. And what they are trying to do is use this health care debate, which is so important to uninsured Americans and others, to advance their anti choice agenda, and that's wrong."
Senators are on their eight straight day of debate on health care legislation. Lawmakers worked through the weekend to debate the bill, and were paid a rare visit by President Obama, who on Sunday rallied his fellow party members and urged them to "finish the job," adding that "The most costly outcome for everyone would be from a failure to finish."
But in the 40-minute closed-door meeting, Obama did not bring up either abortion or the public option, the other hot button issue in the health care debate.
Majority Leader Sen. Harry Reid, D-Nev., said he was not bothered that the president did not address abortion or the public option.
"Progress is being made and that's not just talk," Reid said. "We've made a lot of progress."
With Democratic leaders unsure if the option for government-run health insurance can get the 60 votes needed to pass, lawmakers are working behind closed doors on a compromise proposal that they hope may get more support.
One of the proposals under discussion would involve a national non-profit insurance plan, similar to the one offered to federal employees and Congressional staffers. The plan would be overseen by the Office of Personnel Management, which supervises the Federal Employees Health Benefits Program. But so far, details of the plan remain murky.
The debate over abortion and the kind of public option that should be included in a health care bill reflects the deep divide within Democrats.