The Senate will vote Monday on whether to begin debate on a bill to make abortion legal nationwide, which is the centerpiece of Democrats’ response to Republican-led efforts to remand the issue to the states.

A Republican filibuster is almost certain to derail the bill.

However, the fact that the vote is being held underscores the Democratic Party’s subtly shifting politics on the issue, as abortion rights advocates demand more vocal support from lawmakers and show little tolerance for outliers.

Sen. Bob Casey of Pennsylvania, one of the Senate’s two remaining Democratic opponents of abortion, said he would vote to open debate on the Women’s Health Protection Act on Monday. And, on Tuesday, Rep. Henry Cuellar of Texas, the House’s last anti-abortion Democrat, will face a fierce primary challenge from an abortion rights supporter who has been embraced by progressive lawmakers in part because of her views on reproductive rights.

The Senate bill comes as the Supreme Court is expected to rule this summer on a Mississippi law that would make abortion illegal after 15 weeks. That would be a violation of the 1973 Roe vs. Wade decision, which established a legal right to abortion up to 24 weeks.

The court’s 6-3 conservative majority is widely expected to uphold Mississippi’s law, and some abortion rights supporters are concerned that the court will overturn Roe entirely. As the separate Texas legal challenge proceeds, the court has also allowed Texas to maintain its ban on the procedure once fetal cardiac activity is detected, which is usually around six weeks.

The court’s decision in the Mississippi case could be crucial in determining which party controls Congress next year in the 2022 midterm elections.

“This is a critical time for us to stand up and show women that we support their right to make their own healthcare decisions, including abortion,” said Sen. Patty Murray (D-Wash.), one of the bill’s supporters. “The Republican Party is making a concerted effort to take away a woman’s right to make her own healthcare decisions, from abortion to reproductive care.” Democrats describe their legislation as “codifying Roe,” but it goes much further.

It would prohibit states from enacting abortion restrictions before a fetus is viable outside the womb, which is around 24 weeks, and in post-viability cases when the patient’s life or health is in jeopardy.

It would also put a stop to Republican efforts to stymie abortion access in states by prohibiting policies like waiting periods, ultrasound requirements, or requiring abortion providers to have admitting privileges at a nearby hospital.

Any state regulations governing abortion providers must apply equally to providers of comparable healthcare services.

Meanwhile, conservatives welcome the opportunity to put Democrats on the record on a bill that they believe will be politically unpalatable to independent or conservative-leaning voters. Casey, who campaigned on a promise to oppose abortion, said recent events compelled him to vote yes on Monday.

His office did not respond to follow-up questions about whether Casey would support the legislation’s final passage or whether he has changed his position on abortion rights.

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

The office of Sen. Joe Manchin III (D-W.Va.), the only other Senate Democrat who opposes abortion rights, declined to comment on his voting intentions.

Senators Susan Collins of Maine and Lisa Murkowski of Alaska frequently join Democrats in support of abortion rights legislation. Collins previously stated that she would not support the bill because it goes beyond simply codifying Roe, and she is concerned that it will eliminate protections for health workers who have moral objections to participating in abortion.