One month after the Supreme Court struck down the right to an abortion, Democrats who then controlled the House pushed through a bill aimed to ensure access to contraception nationwide. All but eight Republicans opposed it.

That vote two years ago, opposing legislation that would protect the right to purchase and use contraception without government restriction, may come back to haunt Republicans in November, as they seek to keep hold of their slim majority at a time when real fears about reproductive rights threaten to drive voters away from them.

The risks they face became glaringly clear last week, after the Alabama Supreme Court ruled that frozen embryos should be considered children. In response, a stampede of Republicans in Congress have rushed to voice their support for in vitro fertilization treatment — even though they have supported legislation that could severely curtail or even outlaw aspects of the procedure.

A new national poll conducted by Americans for Contraception and obtained by The New York Times found that most voters across the political spectrum believe their access to birth control is actively at risk, and that 80 percent of voters said that protecting access to contraception was “deeply important” to them. Even among Republican voters, 72 percent said they had a favorable view of birth control.

When voters were told that 195 House Republicans had voted against the Right to Contraception Act, 64 percent of them said they would be less likely to support Republican candidates for Congress, according to the poll. And overall, the issue of protecting access to contraception bolstered voters’ preference for Democrats by nine points, giving them a 12-point edge over Republicans, up from three.

The survey found that birth control access was especially motivating to critical groups in the Democratic coalition, including Black voters and young people, who are currently less enthusiastic about the election.

Pollsters said the shift in overall party preference — known as the generic ballot — was notable, particularly by such a large margin.

“It’s really hard to move a generic ballot because parties are branded,” said Molly Murphy, the president of Impact Research, which conducted the poll. “You can move numbers on named candidates, but people generally think they know the parties. It’s hard to change that perception.”

While the survey, conducted in early February, did not contain questions about I.V.F., its findings may help explain why so many Republicans have distanced themselves from a voting record that promotes policies that could put such procedures at risk.

Speaker Mike Johnson, for instance, added his voice on Friday night to the growing chorus of Republicans claiming they support in vitro fertilization treatments. But like many of the other House Republicans now saying they back unrestricted I.V.F., Mr. Johnson is a co-sponsor of the Life at Conception Act, which would recognize a fertilized egg as a person with equal protections under the 14th Amendment.

The bill states that the term “human being” includes “all stages of life, including the moment of fertilization,” and does not include any exceptions for I.V.F. and fertility treatments. If enacted, that could severely restrict I.V.F. treatments, which typically involve the creation of several embryos, only one of which is implanted while the others are frozen to allow for subsequent attempts at a successful implantation.

It is the latest bit of politically rocky terrain that Republicans have had to walk on issues of reproductive health since the Supreme Court’s 2022 decision to overturn Roe v. Wade, which has made real to voters the threat that other rights could be next to go. According to the new poll, three out of five voters living in states where abortion has been banned or restricted said they were concerned that contraception is next.

Ms. Murphy said Republicans’ reaction to the Alabama ruling indicated that they know they have a political crisis on their hands.

“The reason they’re having to come out against this is because they know that it isn’t plausible for voters to believe it was just a court in Alabama, but more of a representation of what this entire party stands for,” Ms. Murphy said. “If they thought this was an outlier ruling from a rogue court in the South and they didn’t have to say anything, they wouldn’t be saying anything. This is damage control.”

It will be the second national election cycle in which Republicans face a bind of their own making as they try to reconcile their party’s hard-line policies on women’s health — based on a fealty to a conservative religious doctrine — with a vast majority of the country that now views the issue differently.

A majority of voters support the Right to Contraception Act across party, racial and gender lines, according to the poll. About 94 percent of Democrats support it, and 68 percent of Republican voters favor it.

But when the proposal came before the House, Republicans balked. Many of them claimed that they supported contraception in practice but considered the bill a gateway to allowing abortion. They argued that the bill’s definition of contraceptives could be interpreted to include pills that induce abortion.

“The Republican Party has so underestimated the way the country has changed,” said Karen Finney, a longtime abortion rights activist. “This is part of the deal they made with very far-right conservatives who are unbending on these issues. There are Republicans who recognize the damage it could do to their base of support if they were to modify in any direction.”

Susan B. Anthony Pro-Life America, a prominent anti-abortion group, opposed the Right to Contraception Act and rated lawmakers according to their votes on the bill, downgrading those who supported it and rewarding those opposed.

Ms. Finney said Democrats will score their political opponents on it, too, in their own way. “You will see ads in some places questioning whether the Republican Party really is saying ‘abstinence only,’” Ms. Finney said. “That’s not going to win the youth vote.”

Some vulnerable Republicans have already been trying to change course on contraception after opposing the 2022 bill. Representative Mariannette Miller-Meeks, Republican of Iowa, led a group of Republican women in the House last year in sponsoring the Orally Taken Contraception Act of 2023, a bill they pitched as a way to expand access to birth control.

Democrats dismissed the bill — which was notably unopposed by Susan B. Anthony Pro-Life America — as so narrow as to have virtually no effect except as an attempt to mask House Republicans’ hostility to contraception. The bill, which Mr. Johnson has yet to bring up for a vote, would direct the Food and Drug Administration to issue guidance for companies that want to make oral contraception available without prescriptions.

Only two drug companies are actively working to offer birth control over the counter. One of them, Opill, was already approved for sale without a prescription before the legislation was introduced. The other, from Cadence Health, is years into the application process with the F.D.A. and would not necessarily benefit from or need the guidance the bill directs the agency to issue.

The new poll by Americans for Contraception, conducted between Feb. 2 and Feb. 8, included interviews with 1,800 voters.

In their conclusion, the pollsters delivered some unequivocal advice to Democratic candidates before the November elections that could also serve as a strong note of caution for Republicans who have opposed birth control access.

“Don’t shy away from talking about all forms of contraception, including I.U.D.s and emergency contraception like Plan B,” they wrote. “Contraception is popular, and voters want to be the ones making the decisions on what methods they use. They do not draw distinctions between types of birth control, and neither should we.”