Rubio was the only person Romney mentioned as someone he was considering for a vice presidential running mate. Another oft-mentioned possibility -- though not by Romney -- New Jersey Gov. Chris Christie, has been selected as the keynote speaker at the convention in Tampa, Fla.
Two female governors, Susana Martinez of New Mexico and Nikki Haley of South Carolina, who is of Indian descent, are also on the program as speakers, The Washington Post reported.
Romney's rivals for the Republican nomination are, for the most part, being kept in the background. Only former U.S. Sen. Rick Santorum of Pennsylvania will be a convention speaker.
Christie told USA Today he plans to make an "emphatic" argument for the Republican approach to addressing challenges facing the nation and for the concept of shared sacrifice to resolve those challenges.
"I'll try to tell some very direct and hard truths to people in the country about the trouble that we're in and the fact that fixing those problems is not going to be easy for any of them," Christie said.
He said he'll use his experiences in New Jersey to demonstrate that "the American people are ready to confront those problems head-on and endure some sacrifice."