Josh Shapiro, the Democratic attorney general of Pennsylvania, is employing a familiar but risky tactic in that state’s governor’s race: He’s paying for a TV ad that appears intended to help one of his opponents in the Republican primary.

The opponent, a QAnon-linked retired military officer and state senator, Doug Mastriano, is leading the nine-person field by about 10 percentage points, according to the RealClearPolitics average of polls in the race. Mr. Mastriano’s rise has alarmed many Republicans in and outside the state.

Some Shapiro advisers had been toying with the ad maneuver for months, although the leadership of the campaign did not discuss the idea until it was clear that Mr. Mastriano was ahead. Mr. Shapiro is running unopposed in the Democratic primary.

Public polls have shown Mr. Shapiro faring better in a head-to-head matchup against Mr. Mastriano than against the other two leading candidates in the Republican primary, former Representative Lou Barletta and Bill McSwain, a former U.S. attorney.

The ad notes that Mr. Mastriano wants to “outlaw abortion” and is “one of Donald Trump’s strongest supporters.” It continues: “He wants to end vote by mail, and he led the fight to audit the 2020 election. If Mastriano wins, it’s a win for what Donald Trump stands for.”

It ends by asking: “Is that what we want in Pennsylvania?”

The ad went live in six media markets across the state on May 5, three days after the news emerged that the Supreme Court appeared ready to overturn Roe v. Wade.

The 30-second spot is ambiguous enough that Republicans have wondered aloud whether the ploy was meant to help or hurt Mr. Mastriano, whose shoestring campaign is short on cash.

“I’m going to have to send him a thank-you card,” Mr. Mastriano has said. His campaign did not respond to a request for comment.

Will Bunch, a columnist for The Philadelphia Inquirer, has accused the Shapiro campaign of playing a “dangerous game,” arguing that Mr. Mastriano’s support for conspiracy theories makes him a uniquely toxic candidate.

Shapiro’s team is making no apologies. “Both public and private polling indicates that Doug Mastriano is poised to become the Republican nominee on May 17,” said Will Simons, a campaign spokesman. “Our campaign is prepared to start the general election now and make sure Pennsylvanians know his real record.”

Meddling in an opposing primary can backfire, said Mike Murphy, a longtime Republican strategist who opposes Mr. Trump, calling Mr. Shapiro’s move “irresponsible.”

Sam Katz, a three-time former Republican candidate for mayor of Philadelphia, ran ads attacking one of the candidates in that race’s 1999 Democratic primary, hoping it would bolster his preferred opponent, John Street.

Mr. Street would go on to become the second Black mayor in the city’s history, though he won the general election against Mr. Katz that year by fewer than 8,000 votes.

In an interview, Mr. Katz defended the ads bolstering Mr. Street as his least bad option, since the two other top Democrats in that contest were likely to beat him more easily. The two former rivals have since become friends.

The Mastriano ad is “very shrewd” and “essential,” said Mr. Katz, a Shapiro ally. During campaigns, he said, “you won’t get a defining moment but maybe once — and now is that moment.”

Kirsten Noyes contributed research.