Presidential candidate Donald Trump is now taking on one-time Obama "car czar" Steve Rattner.

On Monday, Trump fired off a tweet telling Rattner: "I think you should have gone to prison for what you did, I guess Obama saved you."

He ended the tweet telling Rattner to watch: "I will win!"

It was unclear what activity Trump was referring to that should have landed Rattner in jail. Trump did not respond to CNNMoney's request for comment.

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In 2010, Rattner did pay $10 million in fines when he settled with the New York state attorney general for his alleged involvement in a pension fund scheme. While Rattner was never charged criminally, some others who were involved in the same scheme, such as former New York comptroller Alan Hevesi, were.

Rattner did not respond to requests for comment on the Twitter (TWTR) post about going to prison.

On Monday, Rattner did make an appearance on MSNBC's "Morning Joe" and said Trump would not win the nomination.

"I not only don't think he's gonna be the nominee, I don't think he's gonna run," Rattner said. "He'd have to spend some of his own money to do it. He'd end up losing. He'd end up being a spoiler. I'm not sure that's exactly what he wants to do."

In the past, Rattner has been critical of Trump in his tweets.

In 2009, Rattner worked as the lead auto adviser to the U.S. Treasury Secretary to help General Motors (GM) and Chrysler (FCAU) get out of bankruptcy.