By Steve Holland

ASTON, Pennsylvania (Reuters) - Popular Florida Senator Marco Rubio campaigned with Mitt Romney on Monday, increasing speculation that the conservative Cuban-American might be high on the Republican presidential front-runner's list of potential vice presidential running mates.

Rubio would bring a number of attributes to the ticket should Romney pick him. He could appeal to some Latinos, most of whom back Democratic President Barack Obama, and he could inspire conservatives who worry Romney is too moderate.

But Romney will have to look hard at Rubio's inexperience on the national stage. The Tea Party favorite has been a U.S. senator for less than two years, making him an unlikely match for a role in which he would have to be ready to step into the presidency at any time.

The two men appeared side-by-side at a news conference where Romney did most of the talking. They then held a town hall event where they took turns answering questions from supporters.

Romney brushed off two questions from reporters whether Rubio was on his list for potential vice presidential picks, a process that will play out over the next four months ahead of the Republicans' August convention in Florida to nominate their candidate to face Obama in the November 6 election.

"The process for selecting a vice presidential running mate is just beginning," he said, noting that longtime aide Beth Myers has begun to put together a system for checking out possible nominees, "but we really haven't had a discussion yet of putting together a list or evaluating various candidates."

The process will include "a look at tax returns and things of that nature. So she's putting the process together and it's just at the very early stages."

Rubio, asked whether he had the experience necessary to serve as vice president, "I'm not talking about that process anymore."

Rubio, 40, has lately sounded coy about the idea of a vice presidential nod after ruling it out repeatedly in the past. Conservatives speak warmly of the idea and Romney advisers say he will be considered, but some Republicans voice worries he may simply be too young for the job.

Obama leads Romney among Hispanic voters by a whopping 40 percentage points, putting pressure on Romney to take steps to reduce that margin and make himself more competitive in states where the Latino vote could be decisive, like New Mexico and Nevada.

Romney told reporters he planned to lay out a series of policy proposals aimed at addressing a sore point with the Hispanic community, the lack of a comprehensive immigration reform.

He said he wanted to adjust the U.S. visa program "to make it fit the needs of our country," but was not specific.

"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," Romney said.

Romney has appeared with other potential vice presidential picks, such as Wisconsin Congressman Paul Ryan, Virginia Governor Bob McDonnell, Ohio Senator Rob Portman, New Jersey Governor Chris Christie, South Carolina Governor Nikki Haley and New Hampshire Senator Kelly Ayotte.

The body language between Romney and Rubio was warm and Romney repeatedly handed off the microphone to Rubio to respond to questions. The joint visit was planned when Pennsylvania, which votes in a Republican primary on Tuesday, was expected to be close between Romney and his last conservative challenger Rick Santorum, who has since pulled out of the campaign.

(Editing by Mohammad Zargham)