Author Ed Klein said today on "Fox and Friends" that Bill Clinton sees Sen. Marco Rubio as the biggest threat to his wife becoming president in 2016.

Klein, who just released Unlikeable: The Problem with Hillary, said the information came to him through sources who sat down to dinner with Clinton recently. 

"He went down the entire list of the Republican candidates and he came to the conclusion that Marco Rubio would present Hillary with the greatest challenge because of his age. He represents generational change. He also has a real appeal to the Latino vote. If he can siphon off that vote from Hillary, he could be a real formidable challenger," said Klein, adding that Mr. Clinton believes Rubio needs to be "destroyed before his candidacy gets off the ground."

The 42nd President tweeted a picture of himself watching the former First Lady last night, and later said her performance "showed why she should be President."

Rubio responded later on in the show, saying that he knows people will "take shots" at him, and is motivated by it.

"I'm ready to do whatever it takes, including confronting the Clinton forces to win this election," said Rubio.

Watch the senator's thoughts on the Dem debate in the clip below.