Republican Mitt Romney has declined to endorse an immigration proposal from potential running mate Marco Rubio and says he's looking at the senator's plan.

Standing with Rubio in Aston, Pa., the presumptive Republican presidential nominee didn't come down one way or the other on Rubio's proposal.

Romney also wouldn't talk about running mates. He said he's just starting the process of identifying a vice presidential candidate, and is working with his attorneys and accountants.

Romney answered reporters' questions Monday for the first time since effectively securing the Republicans' presidential nomination.

He also said he supports extending low interest rates on student loans, which President Barack Obama has been pressing Congress to do.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

Mitt Romney on Monday assailed Democratic President Barack Obama's energy polices and urged Pennsylvania voters to back him in the fall. He planned a second stop later in the day with Florida Sen. Marco Rubio.

The Cuban-American, first-term senator is considered a top potential pick for vice president. He's also the latest in a string of possible running mates to campaign with Romney, but is the first to get an audition since former Pennsylvania Sen. Rick Santorum left the race and Romney staffers formally began organizing the process of searching for a No. 2.

"I need your help for Nov. 6. So you need to get your friends to vote," the former Massachusetts governor told about 200 people gathered at a research and development facility at Consol Energy, a Pittsburgh-area Fortune 500 company.

He told employees of the national gas and coal firm that Obama has put the U.S. on a road where "energy is going to become more and more expensive."

"The onslaught of regulations _ holding off on drilling in the Gulf, holding off on drilling of the Outer Continental Shelf, holding off on drilling in Alaska, trying to impose the federal government into fracking regulations, with regards to natural gas, and then, of course, all the regulations relating to coal, making it harder to mine it, making it harder to use it _ these things have made the cost of energy go up," the former Massachusetts governor said.

Santorum's exit left Romney without significant opposition in Pennsylvania's GOP presidential primary Tuesday.

Romney scheduled his first joint appearance with Rubio in Aston, Pa., near Philadelphia. Rubio endorsed Romney on March 29.

The former Massachusetts governor has campaigned with a growing list of possible running mates, starting with New Jersey Gov. Chris Christie, who backed Romney as soon as he definitively decided not to run. Romney campaigned extensively with South Carolina Gov. Nikki Haley before her state's primary in January.

He's also shared the stage with South Dakota Sen. John Thune, Virginia Gov. Bob McDonnell, Wisconsin Rep. Paul Ryan, former Minnesota Gov. Tim Pawlenty, New Hampshire Sen. Kelly Ayotte, and Ohio Sen. Rob Portman. Portman and Ayotte have recently done individual campaign stops for Romney.

The meetings give Romney and his aides a chance to evaluate potential contenders and work with their staffs. In some cases, it's quickly become clear that the operations don't mesh well. In other cases, the rapport between politicians is obvious, as it was with Ryan, who joined Romney before the vote in Wisconsin on April 3.

Rubio's Cuban background, home state of Florida and rising national profile have put him at the top of many vice presidential lists. Still, some Republicans have questioned whether he'd be better off in the running-mate slot or could wait until 2016 to mount his own bid for the top job.

Rubio has said he has no interest the vice president's job and prefers to stay in the Senate.