OVERLAND PARK, Kan. — Kansas voters resoundingly decided against removing the right to abortion from the State Constitution, according to The Associated Press, a major victory for the abortion rights movement in one of America’s most reliably conservative states.

The defeat of the ballot referendum was the most tangible demonstration yet of a political backlash against the U.S. Supreme Court’s decision to overturn Roe v. Wade, the landmark decision that had protected abortion rights throughout the country. The decisive margin came as a surprise, and after frenzied campaigns with both sides pouring millions into advertising and knocking on doors throughout a sweltering final campaign stretch.

At a campaign watch party in suburban Overland Park, abortion rights supporters yelled with joy when MSNBC showed their side with a commanding lead.

“We’re watching the votes come in, we’re seeing the changes of some of the counties where Donald Trump had a huge percentage of the vote, and we’re seeing that just decimated,” said Jo Dee Adelung, 63, a Democrat from Merriam, Kan., who knocked on doors and called voters in recent weeks.

She said she hoped the result sent a message that voters are “really taking a look at all of the issues and doing what’s right for Kansas and not just going down party lines.”

The vote in Kansas, three months before the midterm elections, was the first time American voters weighed in directly on the issue of abortion since the U.S. Supreme Court overturned Roe v. Wade this summer. The referendum, watched closely by national figures on both sides of the abortion debate, took on added importance because of Kansas’ location, abutting states where abortion is already banned in nearly all cases. More than $12 million has been spent on advertising, split about evenly between the two camps. The amendment, had it passed, would have removed abortion protections from the State Constitution and paved the way for legislators to ban or restrict abortions.

“We’ve been saying that after a decision is made in Washington, that the spotlight would shift to Kansas,” said David Langford, a retired engineer from Leawood, Kan., who wants the amendment to pass, and who reached out to Protestant pastors to rally support.

The push for an amendment was rooted in a 2019 ruling by the Kansas Supreme Court that struck down some abortion restrictions and found that the right to an abortion was guaranteed by the State Constitution. That decision infuriated Republicans, who had spent years passing abortion restrictions and campaigning on the issue. They used their supermajorities in the Legislature last year to place the issue on the 2022 ballot.

That state-level fight over abortion limits took on far greater meaning after the nation’s top court overturned Roe, opening the door in June for states to go beyond restrictions and outlaw abortions entirely. The Roman Catholic Church and other religious and conservative groups spent heavily to back the amendment, while national supporters of abortion rights poured millions of dollars into the race to oppose it.

Supporters of the amendment have said repeatedly that the amendment itself would not ban abortion, and Republican lawmakers have been careful to avoid telegraphing what their legislative plans would be if it passed.

“Voting yes doesn’t mean that abortion won’t be allowed, it means we’re going to allow our legislators to determine the scope of abortion,” said Mary Jane Muchow of Overland Park, Kan., who supported the amendment. “I think abortion should be legal, but I think there should be limitations on it.”

If the amendment had passed, though, the question was not whether Republicans would try to wield their commanding legislative majorities to pass new restrictions, but how far they would go in doing so. Many Kansans who support abortion rights said they feared that a total or near-total abortion ban would be passed within months

Abortion is now legal in Kansas up to 22 weeks of pregnancy.

“I don’t want to become another state that bans all abortion for any reason,” said Barbara Grigar of Overland Park, Kan., who identified herself as a moderate and said she was voting against the amendment. “Choice is every woman’s choice, and not the government’s.”

A Pew Research Center survey published last month found that a majority of Americans said abortion should be legal in all or most cases, and that more than half of adults disapproved of the Supreme Court’s decision to overturn Roe.

Kansas has been a focal point of the national abortion debate at least since 1991, when protesters from across the country gathered in Wichita and blocked access to clinics during weeks of heated demonstrations that they called the Summer of Mercy.

At times, the state has seen violence over the issue. In 1986, a Wichita abortion clinic was attacked with a pipe bomb. In 1993, a woman who opposed abortion shot and injured Dr. George Tiller, one of only a few American physicians who performed late-term abortions. In 2009, another anti-abortion activist shot and killed Dr. Tiller at his Wichita church.

In recent years, and especially in the weeks since Roe fell, Kansas has become a haven of abortion access in a region where that is increasingly rare.

Even before the Supreme Court’s action, nearly half of the abortions performed in Kansas involved out-of-state residents. Now Oklahoma and Missouri have banned the procedure in almost all cases, Nebraska may further restrict abortion in the next few months, and women from Arkansas and Texas, where new bans are in place, are traveling well beyond their states’ borders.

Kansas is reliably Republican in presidential elections, and its voters are generally conservative on many issues, but polling before the referendum suggested a close race and nuanced public opinions on abortion. The state is not a political monolith: Besides its Democratic governor, a majority of Kansas Supreme Court justices were appointed by Democrats, and Representative Sharice Davids, a Democrat, represents the Kansas City suburbs in Congress.

Ms. Davids’s district was once a moderate Republican stronghold, but it has been trending toward Democrats in recent years. Her re-election contest in November in a redrawn district may be one of the most competitive House races in the country, and party strategists expect the abortion debate to play an important role in districts like hers that include swaths of upscale suburbs.

Political strategists have been particularly attuned to turnout in the Kansas City suburbs, and are seeking to gauge how galvanizing abortion is, especially for swing voters and Democrats in a post-Roe environment.

“They’re going to see how to advise their candidates to talk about the issue, they’re going to be looking at every political handicap,” said James Carville, the veteran Democratic strategist. “Every campaign consultant, everybody is watching this thing like it’s the Super Bowl.”

As the election approached, and especially since the Supreme Court decision, rhetoric on the issue became more heated. Campaign signs on both sides have been vandalized, police officials and activists have said. In the Kansas City suburb of Overland Park, vandals targeted a Catholic church, defacing a building and a statue of Mary with red paint.

Before the vote on Tuesday, which coincided with primary elections, Scott Schwab, the Republican secretary of state, predicted that around 36 percent of Kansas voters would participate, up slightly from the primary in 2020, a presidential election year. His office said that the constitutional amendment “has increased voter interest in the election,” a sentiment that was palpable on the ground.

“I like the women’s rights,” said Norma Hamilton, a 90-year-old Republican from Lenexa, Kan. Despite her party registration, she said, she voted no.