By JANET ADAMY and GREG HITT

WASHINGTON -- Antiabortion lawmakers in the Senate plan to introduce an amendment as soon as Monday to restrict insurance coverage of abortion in the health bill, setting up a showdown that has no clear path to resolution.

Both sides agree the amendment likely doesn't have enough votes to pass, but antiabortion groups and Sen. Ben Nelson (D., Neb.) say they will continue insisting on tough language as a condition for supporting the overall bill.

The lack of a clear meeting point makes abortion somewhat different from the other top obstacle to the bill's passage, the publicly run insurance plan that some Democrats oppose. There, both sides are weighing a handful of compromises.

On abortion, said Sen. Nelson, "it's certainly not a lock that there's language in the middle."

As it stands now, the Senate health bill would include abortion coverage in the new public plan and would allow women who receive government tax credits for insurance to enroll in a plan that covers the procedure. The tax credit would be segregated so none of it could go toward funding abortion.

Antiabortion advocates say that's too lenient and want to prevent any insurance plan that receives federal funding from covering the procedure, except in the case of rape, incest or danger to the life of the mother

Sen. Nelson plans to introduce an amendment that mirrors the tighter funding restrictions the House added to its health bill, based on a proposal by Rep. Bart Stupak (D., Mich.). The measure would prevent the public insurance plan from covering abortion. It would also prohibit any woman who got the tax credit from enrolling in a plan that covers the procedure.

Sen. Nelson has said he won't vote for a bill that doesn't include these restrictions. With no signs the measure will pass, Sen. Nelson said Friday that "we're going to keep talking" to reach a compromise on the issue. But his antiabortion allies say they won't accept anything less than the Stupak language.

"That's not a negotiation we're prepared to have," said Richard Doerflinger, associate director of the secretariat of pro-life activities for the U.S. Conference of Catholic Bishops, which pushed for the abortion restrictions in the House bill. "I really don't know how you compromise further."

Abortion-rights advocates are taking a similarly rigid line, saying that such a change would represent the biggest setback for them in decades. "As far as I'm concerned, this bill is a compromise," said Cecile Richards, president of Planned Parenthood Federation of America, which supports the bill in its current form. "People are really ready to move on on this issue and not have a protracted debate about legal abortion in this country."

Both sides see the Senate bill as a turning point in the abortion debate and are making a big push to rally supporters. Abortion-rights advocates brought hundreds of supporters to the Capitol last week to meet lawmakers and have been running television advertisements calling the amendment an attempt to roll back women's rights. The Catholic bishops on Friday sent an alert to parishes urging their members to contact their lawmakers in support of Sen. Nelson's amendment, and plan to oppose the bill if they don't win the change.

The abortion fight could prove decisive as Democratic leaders seek to build the 60-vote majority needed to ensure passage of the broader legislation. If Sen. Nelson ultimately decides to oppose the legislation, Democratic leaders will have to look for support across the aisle from Maine's two Republican senators, Olympia Snowe and Susan Collins, both of whom are supportive of abortion rights. If that happens, the Maine contingent would have big leverage to dictate further changes to the bill, especially on the issue of the government-run insurance plan.

Some senators say they regret that abortion has taken on outsize importance in the health bill, which is designed to expand insurance to tens of millions of Americans and slow the growth of health costs. "It is a shock to all of us...that this has come down to an abortion issue," Sen. Patty Murray (D., Wash.) said last week.

Write to Janet Adamy at janet.adamy@wsj.com and Greg Hitt at greg.hitt@wsj.com