WASHINGTON -- Former President Bill Clinton knows just how high the political stakes are in the fight to overhaul America's health care system.

His failed attempt to revamp the delivery of medical care contributed to the Republican takeover of the House and Senate in 1994.

Fast forward to 2009, where health care's white-hot spotlight now shines on the Senate. Clinton is still in the picture, and he's expected to speak to Senate Democrats about health care legislation during their weekly caucus Tuesday, officials said. They spoke on condition of anonymity because they were not authorized to discuss his schedule.

President Barack Obama wants to sign the legislation into law by the end of the year. But abortion opponents in the Senate are seeking tough restrictions in the health care overhaul bill, a move that could roil a shaky Democratic effort.

Sen. Ben Nelson, D-Neb., said he could not support a bill unless it clearly prohibits federal money from going to pay for abortions. Nelson is weighing options, including offering an amendment similar to the one passed by the House this weekend.

"While there may be different views about abortion, I think there's a strong majority against using federal dollars to fund abortions," Nelson said Tuesday on NBC's "Today."

The House-passed restrictions were the price Speaker Nancy Pelosi, D-Calif., had to pay to get a health care bill passed, on a narrow 220-215 vote. But it's prompted an angry backlash from liberals at the core of her party, and some are now threatening to vote against a final bill if the curbs stay in.

Obama said the legislation needs to find a balance.

"I want to make sure that the provision that emerges meets that test -- that we are not in some way sneaking in funding for abortions, but, on the other hand, that we're not restricting women's insurance choices," Obama said in an interview with ABC News.

Senate Democrats will need Nelson's vote -- and those of at least a half-dozen other abortion opponents in their caucus. They face a grueling debate against Republicans who are unified in their opposition to a sweeping remake of the health care system. It's unclear how the abortion opponents would line up; the pressure on them will intensify once the legislation is on the floor.

An intraparty fight over abortion is the last thing Majority Leader Harry Reid needs. Reid, D-Nev., is already facing a revolt among Democratic moderates over the government-sponsored health plan that liberals want to incorporate in the legislation as a competitor to private insurance companies.

Reid, who is himself opposed to abortion, will have to confront the issue directly as he puts together a Democratic bill for floor consideration. The committee-passed Senate versions differ on abortion, but none would go as far as the restrictive amendment passed by the House.

The House bill would bar the new government insurance plan from covering abortions, except in cases or rape, incest or the life of the mother being in danger. That's the basic rule currently in federal law.

It would also prohibit health plans that receive federal subsidies in a new insurance marketplace from offering abortion coverage. Insurers, however, could sell separate coverage for abortion, which individuals would have to purchase entirely with their own money.

At issue is a profound disagreement over how current federal restrictions on abortion funding should apply to what would be a new stream of federal funding to help the uninsured gain coverage.

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AP Special Correspondent David Espo contributed to this report.