The leak of a draft Supreme Court opinion that would overturn the landmark Roe v. Wade abortion decision sent shock waves through America’s political classes Tuesday and threatened to upend the conventional wisdom about the prospects for both parties in the coming midterm elections.

If issued as a formal decision, the draft majority opinion, written by Justice Samuel Alito in February, would end the court’s protection of a woman’s right to an abortion and relegate that authority to legislators in statehouses or the U.S. Congress.

The opinion itself was not entirely unexpected, but the manner in which it became public — via a leak to Politico — was highly unusual. Politicians in both parties seemed caught off-guard, and analysts began rethinking their predictions for close races across the country if the abortion issue becomes a central focus of the fall campaign.

A formal opinion in the case, Dobbs v Jackson Women’s Health Organization, is expected in the next two months. The outcome of the case and the language of the draft opinion could still change before its release.

Republicans generally welcomed the prospect of a decision overturning Roe, but instead of celebrating spent most of the day excoriating the person or persons responsible for the leak.

In a written statement, Senator McConnell called it an “yet another escalation in the radical left’s ongoing campaign to bully and intimidate federal judges.” At a later press conference, he urged reporters to focus not on the draft itself but on the fact that it was leaked.

Democratic strategists professed to be unsure how the issue would play out at the polls. President Biden, when asked about the potential political fallout from what he called a “radical decision,” demurred, stating only that he had not thought through such things.

His office, however, wasted no time in attempting to rally its base around the cause.

“It will fall on voters to elect pro-choice officials this November,” Mr. Biden’s office said in a written statement released moments before his comment. “At the federal level, we will need more pro-choice Senators and a pro-choice majority in the House to adopt legislation that codifies Roe, which I will work to pass and sign into law.”

Vice President Harris leaned in even harder, accusing Republicans of “weaponizing” the law against women and calling on supporters to “fight for women and for our country with everything we have.”

An email sent shortly before 10 a.m. by a political action committee affiliated with Speaker Pelosi suggested that the best response from Democrats to the opinion should be to send money.

“The only — ONLY — way to get justice is to win this election, protect the Democratic House and Senate, and replace every last anti-choice Republican who made this happen,” the email said.

Polling by Gallup on the issue dating back as far as 1975, two years after the decision in the Roe case, has consistently shown that a majority of Americans support some form of abortion access.

Between a quarter and a third of Americans have told pollsters they believe abortion should be legal in all circumstances, Gallup said, and about half support the right under certain circumstances. About 20 percent of Americans believe abortion should be entirely illegal.

A spokeswoman for the Democratic Legislative Campaign Committee, Laurene McIlvaine, said the issue resonates strongly with Democrats and now has the potential to be a key issue for independent voters as well.

“I think abortion rights will be a very mobilizing issue at every single level of the ballot, including state legislative races,” Ms. McIlvaine said.

The 2022 election season kicked off in earnest Tuesday with primary elections in Ohio and Indiana. Eleven other states have primaries on subsequent Tuesdays this month. Republicans had been aiming to make big inroads in both houses of Congress in November given Mr. Biden’s abysmal approval ratings, rampant inflation, and global security concerns.

Abortion rights advocates, among them Planned Parenthood and Naral Pro-Choice America, had already announced that they were funneling some $150 million into the midterm elections in key swing states such as Georgia, Michigan, Pennsylvania, and Wisconsin.

A strategist with the Democratic American Bridge PAC, Julie McClain, said her group is spending $100 million on the midterms and said that, while it is too early to predict the impact of Monday’s leak on the races, her group will be redoubling its efforts.

“Women voters were the driving force behind Democratic victories in 2018 and 2020,” Ms. McClain said. Now “it’s on all of us, and particularly the Democratic Party, to tap into that and communicate that and stoke that fire between now and Election Day.”