Oprah Winfrey’s 25-year run as the queen of daytime talk has been quite amazing. Her “only in America” story has endeared her to fans and made her a force in the nation’s cultural arena, not to mention making her a billionaire. She wraps up her highly rated show this week (reruns will go into September), and the time is clearly right. It’s been a great run, but the Oprah touch is not as magical as it once was, with ratings wobbling and PR potholes becoming more frequent.

Oprah entered the daytime television world at an opportune time, when the boring lineup included soaps and goofy discussion shows such as “Donahue.”

Oprah grew up in the South and broadcast from the Midwest, which surely helped her understand a wide range of viewers. She was a real person who understood the ups and downs of real life, having battled her way out of poverty with an industrious, can-do approach. Oprah was sociable, empathetic, and just as important, not perfect.

These traits helped her reach viewers across all demographic and socioeconomic boundaries. Viewers liked Oprah because they knew her emotions were real.

Oprah’s program had a solid balance of serious and fun topics. She took on challenging social issues with sophistication. Local television affiliates often used the Oprah show as a lead-in to late afternoon newscasts, so it was in their interest to promote Oprah heavily to provide a ratings boost for local news. Oprah’s ratings power made her show the place to be for authors, new products, public affairs leaders and flawed celebrities. The Oprah “brand” was marketed beyond television into magazines, satellite radio, motion pictures and the philanthropic world. She spun off talkers, including Dr. Phil, Dr. Oz and Nate Berkus.

But even the most popular television shows and personalities eventually rust, and Oprah is no exception. Her ratings have slipped by about a third in the past five years, with even “Judge Judy” providing competition. It is likely some conservative and politically independent viewers cooled on Oprah in 2008 when she became a visible political partisan with her active campaigning for then-candidate Barack Obama.

Several PR hits have hurt the Oprah image of infallibility. There has been an abuse scandal at the girls’ school she founded in South Africa. Oprah helped promote books by James Frey and Herman Rosenblatt, both of which later turned out to have serious credibility problems. Occasional Oprah guest, pediatrician Mel Levine, became the subject of abuse charges and recently committed suicide.

Oprah’s cable network, OWN, premiered earlier this year, more than a year behind the target launch date. It has sputtered with disappointing ratings and un-Oprah-like programming. In a recent published interview, Oprah admitted the channel struggles. “It’s not where I want it to be,” she said.

Oprah’s departure from daytime television leaves a void, and programmers are scrambling to find a replacement personality. Of course, replacing Oprah’s magic will be impossible. Anderson Cooper has a talk show ready to debut this fall, but let’s face it, audiences already know him, and his ratings on CNN are dreadful.

Katie Couric appears headed to a daytime talk show, but it will be fall 2012 before it gets going, and the Oprah audience will have already dispersed.

A number of current Oprah affiliates are simply planning to start local newscasts earlier. They know, like Oprah’s fans, that Oprah can’t be replaced.

Jeffrey M. McCall is a professor of communication at DePauw University in Greencastle, Ind., and author of “Viewer Discretion Advised: Taking Control of Mass Media Influences.”