Rubio, who went from being a long shot for the GOP nomination a year ago to leading Crist by a wide margin in recent polls, said in an appearance on "Fox News Sunday" no one knows what Crist's platform is "and we never will."
"You are never going to be able to hold him accountable to anything, because his opinions are going to change based upon what the polling tells him or his political convenience tells him," Rubio said.
Crist's decision to instead run as an independent sets up a three-way race with Democratic U.S. Rep. Kendrick Meek, the third major candidate.
Rubio, a conservative Republican, said his views are "mainstream positions" while those of his opponents "quite frankly put our country on a path that most Floridians don't want us on."
"Conservative thought is the mainstream position in America," Rubio said. "What is not mainstream in America is the belief that we should spend money that we don't have. What's not mainstream in America is the belief that we should take this crazy complicated tax code and make it crazier and more complicated. What is not mainstream in America is this belief that somehow if America weakens its position in the world, the world will become a safer place."
On the issue of immigration reform, Rubio said Arizona's go-it-alone approach "is not the ideal way to deal with a very serious public safety and immigration situation."
"The best way to deal with this is for the federal government to do its job and secure its border with Mexico," Rubio said.
On the Gulf of Mexico oil disaster, Rubio said getting the leak under control is the top priority and then figure out why it happened.
"Ultimately, America still has an energy independence problem and an energy independence goal and an energy dependence problem. And it has to have all of its energy resources at its disposal," he said.
Asked about reports the Internal Revenue Service is investigating his tax records to see if he misused a Republican Party credit card for personal expenses, Rubio said he had reimbursed the party for any such expenses. He said he was "not aware of any investigation" and neither he nor his staff had been in contact with federal authorities.
Rubio also said he stands by his proposal to change the retirement age and cut Social Security benefits for those who are now under age 55.