Asked about news reports that there is a federal criminal investigation into the use of American Express cards issued to him and other elected officials by the Republican Party of Florida, Rubio said that he has not been contacted by law enforcement authorities.
"I'm not aware of any such thing," said Rubio. "The only thing I've ever seen about it is some anonymous source quoted in the newspaper."
Rubio said that he would be surprised to find out that the IRS and FBI were looking into his credit card use. He said that any use of party funds has been repaid and that his use of party credit cards for personal uses was never intentional.
Rubio said he is running for Senate as the candidate who is willing to level with the American people about adjustments that are needed to preserve Social Security, the retirement program for Americans over 65, and Medicare, the government health care program for senior citizens. Rubio has said that adjustments would be limited to Americans under a certain age, such as 55.
Although Democrats are planning to hammer him on the issue in a state with a large senior citizen population, Rubio said he is not afraid to touch the proverbial "third rail" of American politics.
During his interview with "Nightline," Rubio said that the Medicare prescription drug program may have to be revisited for members of his generation. Rubio is 38.
"[T]he fact of the matter is there may not be a prescription drug program for members of my generation," said Rubio.
The changes that Rubio envisions to the prescription drug program are part of a broader set of entitlement reforms that he thinks are necessary to avoid financial insolvency.
"People of my generation are going to have a very different Social Security and Medicare plan than our parents and grandparents have," said Rubio. "That's a fact. Or those programs will go bankrupt on their own and ultimately bankrupt our country."
While being open to ending the prescription drug benefit for younger Americans, Rubio said the program must be maintained for seniors currently on Medicare.
"You cannot go to someone who is 65 or 70 years of age and tell 'em, 'This program that you retired relying on is now being pulled out from underneath you,'" said Rubio.
When Rubio was speaker of the Florida House, he said, "This nation and ultimately the world is headed towards emission caps."
Asked about those comments, Rubio said that he believed them at the time because the presidential nominees of both major parties supported them. He now believes that emission caps are not inevitable and he does not support them.
"When the nominees of both respective parties support cap-and-trade, it appears inevitable that they're going to try to pass a cap-and-trade system in America," said Rubio. "Now luckily, since then, Sen. [John] McCain has changed his position, and this administration has not pushed as hard on it as we feared they might early on in their administration, 'cause they've focused on health care and other issues. But I have never believed that we should implement cap-and-trade in Florida, or in America, and I've never supported big government mandates on energy, because they don't work. And it's only lasting and real impact will be on the economic growth of our country."
Even though Rubio is still in the middle of a Senate race, some Republicans have begun touting him as a possible vice presidential candidate in 2012. The telegenic Rubio is seen as a potentially strong national candidate at some point because of his Latino background, Florida roots and conservative image.
Asked if he would serve as the GOP nominee's running mate in 2012, Rubio said, "No. ... First of all, it's not gonna happen. Second of all, I'm running. ... I really just wanna serve in the Senate for a number of years, you know, handful of years, and then come home and live under the laws we help pass or repeal. That's my goal."
ABC News' Melinda Arons contributed to this report.