AP Photo/Carlos OsorioMarco Rubio.

If Sen. Marco Rubio is concerned about Donald Trump's readiness to assume control of the US nuclear codes, he's not willing to talk about it anymore.

In a Tuesday CNN interview, Trump's former presidential rival stood by his criticisms of the mogul. But he maintained that the presumptive Republican presidential nominee was a positive alternative to Democratic presidential frontrunner Hillary Clinton.

"I'm not going to sit here right now and become his chief critic over the next six months, because he deserves the opportunity to go forward and make his argument and try to win," Rubio said.

Indeed, Rubio is far from the only candidate to awkwardly reverse course on negative statements about the inflammatory presidential candidate. Former Trump rivals such as Sen. Rand Paul, former Louisiana Gov. Bobby Jindal, and former Texas Gov. Rick Perry, among others, have found themselves walking back strongly worded criticisms of Trump.

Trump on Thursday will be trying to swing another prominent Republican who has yet to support the magnate — House Speaker Paul Ryan.

Here are 9 prominent Republican politicians who have reversed course on Trump:

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Former Louisiana Gov. Bobby Jindal

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In a September op-ed for CNN, then Republican presidential candidate Jindal described Trump as "a shallow, unserious, substance-free, narcissistic egomaniac."

"We can decide to win, or we can be the biggest fools in history and put our faith not in our principles, but in an egomaniac who has no principles," Jindal wrote.

But following Trump's victory in the Republican presidential primary, Jindal offered a very tepid endorsement of the real-estate magnate.

"I think electing Donald Trump would be the second-worst thing we could do this November, better only than electing Hillary Clinton to serve as the third term for the Obama administration's radical policies," Jindal wrote in The Wall Street Journal.

Former Texas Gov. Rick Perry

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During his short-lived 2016 presidential bid, Perry called Trump a "cancer on conservatism" and criticized his inflammatory rhetoric about Mexican immigrants.

"Demeaning people of Hispanic heritage is not just ignorant. It betrays the example of Christ," Perry said in his September concession speech. "We can enforce our laws and our borders, and we can love all who live within our borders, without betraying our values."