In an interview that aired on the "Today" show Monday, the 19-year-old hockey hunk who split from Palin's daughter, Bristol, earlier this year also said his would-be mother-in-law resigned from her post as Alaska governor to cash in on her fame.
"I really don't think I'd vote for her if she ran for president," Levi said. "She's very smart, but I just don't think she can handle the stress level as governor. I don't think she can handle it as president."
Johnston indicated that Palin lost her luster for the state post when after her failed bid as John McCain's running mate on the Republican presidential ticket last year. She returned to Alaska "stressed out" and "quiet."
"She wasn't as outgoing," Johnston said. "I didn't see the spark in her eyes about being governor anymore."
She missed the attention that being a figure on the national political stage afforded her -- including the phalanx of photographers that followed her every move and the cabal of Secret Service that traveled with her, he said.
Johnston said cashing in on her fame was not the only reason Palin made her stunning resignation -- stress was also a factor -- but it played a significant role in her decision to leave the post, which will take effect at the end of the month.
"That's not the only reason ... but it's definitely part of it," he said.
The high school drop out, who spent time living with the Palins last winter, said last week that he overheard Palin talking about bailing on Alaska to reap the cash that awaited her.
"She had talked about how nice it would be to take some of this money people had been offering us and you know just run with it, say 'forget everything else,'" he said last week.
A spokeswoman for Palin shot down the claim.
"Absolutely not," a Palin spokeswoman told The Washington Post last week. "She is taking a leap of faith that all will be well personally. This is about what is best for Alaska and not what is best for her personally."