In the ad, Bartoshevich describes herself as a "proud Hillary Clinton Democrat" who is switching sides to vote for a Republican for the first time in her life.
"She had the experience and judgment to be president," she says in the ad. Of McCain, she adds: "I respect his maverick and independent streak, and now he's the one with the experience and judgment. A lot of Democrats will vote McCain. It's OK, really!"
McCain's campaign said the ad was running in Wisconsin and other key states.
Bartoshevich was elected as a Clinton delegate to the convention. After Clinton conceded the race, Bartoshevich said she could not support Obama and threw her support to McCain. The state party stripped her of her status as a delegate last month.
McCain seized on her story, meeting with Bartoshevich for coffee when he campaigned in Racine last month.
On Monday, the mother of two joined other McCain supporters in Denver, outside the convention hall where she had hoped to spend the week.
Wisconsin Democrats said her story is unique and that few other Clinton supporters are backing McCain.
"I'm amazed at how much attention is being given to one solitary person when I have found nobody who has expressed similar sentiments post-primary," said Rep. Tammy Baldwin, a Wisconsin Democrat who supported Clinton during the primaries.
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