Pennsylvania’s race for governor takes on huge stakes for abortion rights.

Now that the Supreme Court has struck down Roe v. Wade, the most important election this year in America when it comes to abortion will be the contest for governor of Pennsylvania.

Josh Shapiro, the state’s Democratic attorney general, is facing off against Doug Mastriano, a Republican state senator who has vowed to make abortion illegal. If Mr. Mastriano wins, the Republican-controlled Pennsylvania legislature is all but certain to move to undo the state’s existing law allowing abortion.

“Roe v. Wade is rightly relegated to the ash heap of history,” Mr. Mastriano said on Friday. “As the abortion debate returns to the states, Pennsylvania must be prepared to lead the nation in being a voice for the voiceless.”

Mr. Shapiro denounced the ruling. “The stakes in this governor’s race could not be more clear,” he said. “The contrast between me and my dangerous opponent could not be greater.”

Nowhere else is a governor’s race so pivotal. In Wisconsin, where the Republican-led Legislature has battled with Gov. Tony Evers, a Democrat who is seeking re-election, a pre-Roe law forbidding abortion automatically went back into effect after Friday’s decision. Mr. Evers has pledged to fight for abortion rights, but he faces a wall of opposition from Republican state legislators.

This week, Mr. Evers ordered Wisconsin’s lawmakers to the State Capitol in Madison for a special session meant to reverse an 1849 law outlawing abortion. Republicans ended the session on Wednesday without taking action.

In Michigan, Gov. Gretchen Whitmer, a Democrat, has backed a series of creative legal arguments to block the state’s 1931 law outlawing abortion from taking effect. In May, a state judge ruled that the law would not immediately go into effect after an eventual Supreme Court ruling on Roe.

Ms. Whitmer has also supported an effort to place a referendum on the November ballot to enshrine abortion rights in Michigan’s Constitution.

Three other states will have questions about abortion decided directly by voters in November.

Kansas and Kentucky have referendums asking voters to affirm that their state constitutions do not guarantee a right to abortion. In Vermont, the ballot will contain a question that would enshrine a person’s right to control their own reproductive choices in the state’s Constitution.

Gov. Laura Kelly of Kansas, a Democrat who supports abortion rights, faces a difficult re-election bid. Her likely Republican opponent, Derek Schmidt, the state’s attorney general, opposes abortion rights.

After Friday’s ruling, Republican governors praised the decision and sought to press the party’s advantage. In Virginia, Gov. Glenn Youngkin said Friday that he would seek a ban on abortion after 15 weeks — though such a move is unlikely to be successful given that Democrats control the State Senate.

“Virginians want fewer abortions, not more abortions,” Mr. Youngkin said. “We can build a bipartisan consensus on protecting the life of unborn children.”

Virginia’s next round of state legislature elections won’t take place until 2023; Mr. Youngkin, who took office in January, is prohibited from seeking a second consecutive term.

Gov. Phil Bryant of Mississippi, a Republican whose state capital was the origin of Friday’s Supreme Court case, said state lawmakers would exercise a “moral duty to protect life at all stages.”

“The pro-life movement also understands that our fight is just beginning,” Mr. Bryant said. “In the coming days, our efforts to assert the full dignity of every human life will become more important.”

Some Republicans minimized the significance of the ruling even as they cheered it. Mr. Mastriano, speaking in Binghamton, N.Y., where he appeared alongside and endorsed Andrew Giuliani in New York’s Republican primary for governor, called the political furor a distraction.

“Sadly, the other side wants to distract us about, you know, Jan. 6,” said Mr. Mastriano, who chartered buses for his supporters to attend the rally that led to the Capitol attack. “Or they want to distract us about Covid. Or distract us about, you know, Roe v. Wade.”

Democratic governors cast the Supreme Court’s decision as a catastrophic move — and the first step toward a broader rollback of women’s rights.

Gov. J.B. Pritzker of Illinois said he had pressed President Biden during his recent visit to Chicago to be more forceful in defending abortion rights. He said Illinois, which is surrounded by states where abortion is illegal or is likely to be outlawed soon, had “a special obligation” to make abortion accessible not just to its citizens but also to visitors.

“We’re an island in the Midwest, in the country, all around us are anti-choice legislatures and state laws and governors,” Mr. Pritzker said in an interview on Friday. “The only thing that will allow us to reverse the terrible direction things are going is electing pro-choice Democratic governors, pro-choice Democratic legislators.”

Democratic candidates for governor in states with Republican-controlled legislatures like Georgia, Arizona and Texas said they would fight for abortion rights if elected — though in practice there is little they could do toward that goal given Republican opposition.

“I will work with the legislature to reverse the draconian law that will now rule our state,” said Stacey Abrams, the Democrat running for governor of Georgia.

Neil Vigdor contributed reporting.