WASHINGTON — Senate Republicans on Monday blocked consideration of a bill to make abortion legal nationwide, the centerpiece of Democrats’ response to Republican-led efforts to ask the Supreme Court to send the issue back to states to decide.

As expected the procedural vote to start debate fell along largely along party lines, 46-48.

But the fact that the vote was held despite its low odds of success underscored the subtly shifting politics in the Democratic Party on the issue, as abortion rights advocates demand more vocal support from lawmakers and show little tolerance for outliers.

One of the Senate’s two remaining Democratic opponents of abortion, Sen. Bob Casey of Pennsylvania, supported Monday’s vote to open debate on the Women’s Health Protection Act.

And on Tuesday, the House’s last anti-abortion Democrat, Rep. Henry Cuellar of Texas, faces a sharply competitive primary from an abortion rights supporter who has been embraced by progressive lawmakers in part because of her views on reproductive rights.

The vote was the Senate’s first “on a stand-alone bill to proactively codify Roe,” said Senate Majority Leader Charles E. Schumer, D-N.Y. “Abortion has never been more at risk in America.”

One Democrat, Sen. Joe Manchin III of West Virginia, voted with Republicans in Monday’s vote. Three Republicans and three Democrats did not vote.

The Senate bill comes as the Supreme Court is expected to issue a ruling this summer on a Mississippi law that would prohibit abortion at 15 weeks. That would violate the court’s 1973 Roe v. Wade decision that established a legal right to abortion up to about 24 weeks.

The court’s 6-3 conservative majority is widely expected to uphold the Mississippi law and some abortion rights supporters worry the court may overturn Roe entirely.

The court has also allowed Texas to keep in place its ban on the procedure once fetal cardiac activity is detected, often at about six weeks, as the separate Texas legal challenge proceeds.

The court’s ruling in the Mississippi case could play a central role in the 2022 midterm elections to determine which party controls Congress next year.

“This is a very important time for us to stand up and show women we are for them and their ability to make their own health care choices, including abortion,” said Sen. Patty Murray, D-Wash., one of the bill’s proponents. “There is a very concerted effort on the Republican side to take away a woman’s right to make her own health care choices from abortion to reproductive care up and down the line.”

Democrats frame their legislation as “codifying Roe,” though it would go further.

It would prohibit states from enacting restrictions on abortion before a fetus is viable outside the womb, or about 24 weeks, and in postviability cases when the patient’s life or health is at risk.

It would also curtail GOP efforts to enact roadblocks to abortion access in the states by prohibiting policies such as waiting periods, ultrasound requirements or demands that abortion providers have admitting privileges at a nearby hospital.

Any state regulations on abortion providers would have to apply equally to providers of similar health care services.

The bill’s broad reach prompted the Senate’s two Republicans who often support abortion rights, Sens. Susan Collins of Maine and Lisa Murkowski of Alaska, to oppose it.

Both senators expressed concern that it could eliminate protections for health workers who have moral objections to participating in abortion and would override state laws such as parental notification of minors who are seeking an abortion.

Murkowski blasted the bill as a partisan measure and blamed Schumer for not trying to find compromise.

“I have long supported a woman’s right to choose, but my position is not without limits, and this partisan Women’s Health Protection Act simply goes too far,” she said. “The fact that my choice is between this bill, or nothing at all, shows how insincere Majority Leader Schumer is about protecting women’s rights. Failing to conduct any outreach and reducing this important issue to nothing more than a designed-to-fail show vote is a disservice to women across America.”

The two Republicans introduced their own, more narrow measure that would prohibit states from imposing an undue burden on patients who choose abortion before fetal viability, or about 24 weeks. It would continue to allow states to enact their own regulations and attempts to strengthen what would prompt an “undue burden,” an issue that the courts have debated for decades.

Conservatives, meanwhile, said they welcome the chance to put Democrats on the record on a bill that they say will be politically unpalatable with independent or conservative-leaning voters.

“Quite frankly, I’m happy that (Senate Majority Leader) Chuck Schumer called for a vote,” said Carol Tobias, president of the National Right to Life Committee. “We want these senators on record so that we can go back to their states and say, ‘Do you know what he just voted for?’ ... And you can be sure we are going to use it as much as we can in the elections this fall.”

Casey, who came into office pledging to oppose abortion, said recent events prompted him to support Monday’s vote.

“Given the recent Supreme Court rulings, potential rulings this year, and the Republican Party’s clear and unrelenting use of this issue as a political weapon, I will vote ‘yes’ to allow debate on this bill,” he said in a recent statement.

His office did not answer follow-up questions on whether Casey would have also supported final passage of the legislation, had it advanced, or whether he has changed his position on abortion rights.

Casey probably would have faced opposition from progressive groups had he not supported Monday’s vote to begin debate.

“Our movement has made it politically not feasible to not support abortion access anymore,” said Morgan Hopkins, director of political strategies at the abortion rights group All Above All, pointing to Casey and Cuellar.

The House approved the bill in September, 218-211, with support from all Democrats except Cuellar. All Republicans opposed it.