Florida Republican Sen. Marco Rubio said Sunday he doesn't believe President Trump would actually fire special counsel Robert Mueller or Deputy Attorney General Rod Rosenstein over frustration with the Russian investigation, despite speculation he's considering such a move.

Asked on CNN's "State of the Union" how he would react if the president fired either of them, Rubio replied: "Well, first of all, that's not going to happen. I don't believe it's going to happen.

"And here's what I would say: The best thing that could happen for the president and the country is a full and credible investigation," he said. "I really, truly believe that. If we want to put all this behind us, let's find out what happened, let's put it out there, and let's not undermine the credibility of the investigation."

Pressed by anchor Jake Tapper if he could have predicted Trump was going to fire former FBI Director James Comey, Rubio said: "Well, I don't think anybody saw that coming, but I don't think that has in any way impeded the work of the FBI on these matters."

"And, in fact, I know it hasn't and they testified so publicly as well and it hasn't impeded the work of the Senate Intelligence Committee. So, you know, these investigations continue to move forward," Rubio said.

Last week, a friend of the president, Chris Ruddy of Newsmax Media, said he believed Trump was considering firing Mueller.

Trump also responded on Friday to news reports he's being investigated for obstruction of justice over his firing of Comey by taking aim at Rosenstein, who wrote a memo recommending Comey's firing. Rosenstein, who appointed Mueller as special counsel, took over control of the Russia investigation after Attorney General Jeff Sessions recused himself.

"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt," Trump tweeted Friday.