Oprah Winfrey is supporting Fetterman over Oz in Pennsylvania’s Senate race.

Dr. Mehmet Oz owes much of his fortune and no small amount of his fame to Oprah Winfrey.

But Ms. Winfrey, who branded Dr. Oz as “America’s Doctor” on her famed television show and went on to co-produce a spinoff, “The Dr. Oz Show,” announced her support on Thursday for her protégé’s Democratic rival, John Fetterman, in the tightly contested U.S. Senate race in Pennsylvania.

“If I lived in Pennsylvania, I would have already cast my vote for John Fetterman for many reasons,” Ms. Winfrey said during a virtual midterms-focused event, according to a clip shared by the Fetterman campaign late Thursday.

A high-profile liberal who endorsed Barack Obama in 2008 and offered public support for Joseph R. Biden Jr. in 2020, Ms. Winfrey had appeared as though she would sit out announcing her views on the Pennsylvania race. Last year, after Dr. Oz declared his candidacy, leaping into politics from 13 years as the celebrity host of his medical-advice show, Ms. Winfrey offered a noncommittal statement that “it’s up to the residents of Pennsylvania to decide who will represent them.”

Dr. Oz, who was a prominent cardiothoracic surgeon in New York in the early 2000s, became a regular guest offering health advice on “The Oprah Winfrey Show” over five seasons. Ms. Winfrey’s company, Harpo Productions, helped create Dr. Oz’s own daytime show in 2009.

The Fetterman campaign on Friday called the Oprah comments “a November surprise,” suggesting they could spur many voters. Although Ms. Winfrey’s endorsement of Mr. Obama in the 2008 Democratic presidential primary is sometimes said to have helped him defeat Hillary Clinton, Ms. Winfrey’s comments in a 2022 Senate race would likely carry far less weight.

On Friday, a spokesman for Mr. Fetterman, Joe Calvello, called the endorsement “a devastating rebuke.” The communications director for the Oz campaign, Brittany Yanick, said “Doctor Oz loves Oprah and respects the fact that they have different politics. He believes we need more balance and less extremism in Washington.”

The Senate race in Pennsylvania is one of several critical contests on Tuesday that could determine which party will control the chamber. Dr. Oz, the Republican nominee, has significantly gained on Mr. Fetterman in the polls since this summer, leaning heavily on messages about public safety and the economy.

Polling by The New York Times and Siena College last week found Mr. Fetterman, the state’s lieutenant governor, up slightly against Dr. Oz, by a 49-percent-to -44-percent margin. But the poll was largely conducted before a debate showing by Mr. Fetterman, who had a stroke in May, that raised Democratic anxieties. (His doctor said recently, adding that Mr. Fetterman has “no work restrictions.”)

Dr. Oz’s politics were largely unknown through his long years in the television limelight, thought he took liberal positions on abortion and gun control that he later disavowed in the Republican primary race for Senate. He won the endorsement of former President Donald J. Trump, who cited his potential appeal to suburban women who had watched him for years on TV.

Ms. Winfrey made her comments on the Pennsylvania race during a virtual event for OWN Your Vote, part of the Oprah Winfrey Network, which aims to provide “Black women with tools and resources to overcome voter suppression in the November election,” according to its website. Dr. Oz has struggled to connect with Black voters in Philadelphia, who are a crucial Democratic constituency.

During the virtual event, Ms. Winfrey also warned that “if we do not get fired up,’’ the wrong elected officials would be in position to make “decisions about how we care for our bodies, how we care for our kids, what books your children can read, who gets protected by the police and who gets targeted.” She ticked off a list of Democrats vying in other close races for Senate in North Carolina, Wisconsin, Nevada and Georgia, and she said she would have supported each of them if she were registered to vote in the state.

“John doesn’t seek endorsements at all, he’s focused on the voters,” said Neil Makhija, the executive director of the group Indian American Impact and a friend of Mr. Fetterman’s, who confirmed his involvement in outreach to Ms. Winfrey’s allies. “But I think the campaign certainly recognized, if there was one endorsement that would be particularly powerful, it was Oprah’s.”