Marco Rubio. AP Photo/Carlos Osorio 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: