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

Oprah Winfrey was instrumental in the rise of Dr. Mehmet Oz, the celebrity television doctor and Republican Senate nominee in Pennsylvania. But now, she says she would support his Democratic opponent, Lt. Gov. John Fetterman, in the tightly contested race.

“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.

She began featuring Dr. Oz as a regular guest almost two decades ago on her hugely popular daytime talk show. Mr. Fetterman’s campaign welcomed Ms. Winfrey’s comment, saying that it “speaks volumes.”

The Senate race in Pennsylvania is one of several critical contests on Tuesday that could determine which party will control the chamber. Dr. Oz 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 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.

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.