Rubio spoke to Fox News' Neil Cavuto the day after Crist cited racism as the reason he left the Republican Party.
Crist became an independent in 2010, during a battle with Rubio for the Republican Senate nomination. He lost to Rubio in Rupublican primary and the general election, where he ran as an independent candidate. Crist joined the Democratic Party in 2012 and announced his run for governor a year later.
On Tuesday, Crist said he made the change because of Republican attacks on Obama.
"I think it's ridiculous and silly," said Rubio. "Two points I would make. First of all, I'm even cautious to even dignify that with a serious response. My prediction is that by the end of this election, even Democrats will be embarrassed that Charlie Crist became a Democrat. I would say that if this was in fact how he felt at the time, why didn't he say it?"
Recent polls have shown Crist leading Republican Gov. Rick Scott by 10 points, with more than half of Florida voters saying Scott does not deserve a second term. Rubio said that Crist was the leader in early polls during their Senate race as well.
"I think once people start to focus in and they realize it's the same Charlie Crist that's running, that he is the same guy that's been basically on every side of every single issue before the country and the state, I think those numbers will change dramatically," Rubio said.
The senator said he has not met any Republican activists who appear to be motivated by racism.