WASHINGTON — Abortion opponents in the Senate are seeking tough restrictions in the health-care overhaul bill, a move that could roil a shaky Democratic effort to pass President Barack Obama's signature issue by year's end.

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

"I want to make sure something comparable ... is in there," Nelson said.

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.

Former President Bill Clinton, whose failed effort to revamp the health care system contributed to the Republican takeover of the House and Senate in 1994, was expected to speak to Senate Democrats about health care legislation during their weekly caucus on Tuesday, officials said. They spoke on condition of anonymity because they were not authorized to discuss his schedule.

An intraparty fight over abortion is the last thing that 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.