ASTON, Pa. (AP) — Mitt Romney declined Monday declined to endorse an immigration proposal from potential running mate Marco Rubio. Romney said he's considering the freshman Florida senator's plan to help some young people stay in the country legally while denying them an opportunity to become citizens.

The presumptive Republican presidential nominee also said he supports a temporary extension of lower student loan interest rates. Democratic President Barack Obama has been pushing Congress for that extension and planned a three-state swing this week to warn students of the potential financial catastrophe they will face if Congress fails to act.

"I think young voters in this country have to vote for me if they're really thinking of what's in the best interest of the country and what's in their personal best interest," Romney said as he stood next to Rubio and answered reporters' questions for the first time since effectively securing the GOP presidential nomination.

Romney refused to say if the Cuban-American senator is on his list of potential vice presidents.

Romney's answers illustrate the careful line he has to walk as he transitions from a brutal Republican primary and into the general election, where he'll have to tussle with Obama for support from the Hispanic, women and young voters who propelled the Democrat to victory in 2008.

Obama, meanwhile, has to hang on to those constituencies. His Tuesday-Wednesday tour through North Carolina, Colorado and Iowa is intended to rally the young supporters he needs again in November.

Romney's language on loans, for example, was distinctly different from when he was last asked about the issue, before the Illinois primary on March 20. Then, he told a young woman concerned about student debt to "by the way, get ready for President Obama's claim. I know he's going to come up at some point and talk about how he's going to make it vanish. And that's another, 'Here I'll give you something for free.' And I'm not going to do that."

Romney also tacked to the right on immigration during the primary. In recent days, he's been highlighting Hispanic concerns at events while leaving out much of the rhetoric he embraced earlier this year. He said Monday that he would outline additional changes to the immigration system in the coming months, particularly with the visa system that governs who is allowed to work in the U.S.

"I anticipate before the November election we'll be laying out whole series of policies that relate to immigration and obviously our first priority is to secure the border, and yet we also have very substantial visa programs in this country," Romney said. "How we adjust our visa program to make it fit the needs of our country is something I'll be speaking about down the road."

Still, he wouldn't go so far as to embrace Rubio's immigration proposal, saying only that it has "many features to commend it." The freshman Florida senator has said his goal is to craft a Republican compromise on the so-called DREAM Act that Romney could support.

The DREAM Act, which has languished on Capitol Hill, would provide a path to citizenship for some young illegal immigrants who attend college or serve in the military.

Rubio is working on a plan that would allow young illegal immigrants brought to the U.S. by their parents to apply for non-immigrant visas. They would be allowed to stay in the country to study or work, could obtain a driver's license but would not be able to vote. They later could apply for residency, but would not have a special path to citizenship.

The Cuban-American senator is considered a top potential pick for vice president. He's the latest in a string of possible running mates to campaign with Romney, and 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.

Romney declined Monday to say if Rubio was on his list of vice presidential candidates. He said his campaign is still setting up the infrastructure that's required to scrutinize potential nominees, including hiring legal and accounting staff.

Romney has put longtime adviser Beth Myers in charge of the search process.

"Beth Myers has begun to put together a number of the names and criteria and so forth that would be associated with that process, but we really haven't had a discussion yet of putting together a list or evaluating various candidates," Romney said. "That's a process. We're looking at various people, resources to help with that process."

The former Massachusetts governor also refused to say whether Rubio is experienced enough to serve as his No. 2. Romney often criticizes Obama, who was a first-term senator when he was elected president, as a "nice guy" who is "in over his head," implying that the Democratic incumbent didn't have the experience he needed for the job.

"I don't think I have any comments on qualifications for individuals to serve in various positions in government at this stage. That is something that we're going to be considering down the road," Romney said.

Rubio said he was no longer commenting on the process. He has said he doesn't want the running-mate slot, and some Republicans have urged him to consider running for the top slot in 2016.

Romney, meanwhile, 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.