WAS IT an audition for the vice-presidential spot?

On the cusp of the Pennsylvania primary, presumptive Republican presidential nominee Mitt Romney campaigned in Delaware County Monday afternoon, accompanied by one of his most-discussed potential running mates, Florida Sen. Marco Rubio.

Rick Santorum's decision to drop out two weeks ago has drained Tuesday's GOP presidential primary of any remaining drama, so Rubio created a buzz as he and Romney answered questions during a town-hall meeting at Mustang Expediting, a transportation company in Aston.

Neither Romney nor Rubio would take that bait at a brief news conference before the event, but the crowd of several hundred appreciated the Florida senator's star power. He drew louder applause than Romney at several points.

Romney, who has called President Obama too inexperienced, declined to answer a Fox News reporter's question about whether Rubio, 40, in his first Senate term, was qualified to serve a heartbeat away from the presidency.

"I don't think I have any comments on qualifications for individuals to serve in various positions in government in this stage," Romney said. "That's something we're going to be considering down the road as we consider various potential vice-presidential nominees."

For his part, Rubio smiled and said, "I'm not talking about that process anymore." In a forum last week, he had said, "If I do a good job as vice president" - before apologizing and correcting himself to say "as a senator."

Republicans in four other Northeastern states are also voting Tuesday: Delaware, New York, Connecticut and Rhode Island. Since Romney has effectively clinched the nomination, he has been seeking to appeal to a broader audience that he will need to win the general election, and the "veepstakes" speculation has grown louder.

Rubio, a favorite of antitax tea-party activists, is the first potential running mate that Romney has campaigned with since sewing up the nomination. Many GOP strategists believe that adding the Cuban-American to the ticket could shore up Romney's standing among Hispanics, the fastest-growing bloc of swing voters. That could help him in battleground states such as Pennsylvania, where Hispanics have nearly doubled since 2000.

On Monday, Rubio described how his parents came from Cuba five decades ago with little education or knowledge of English and became successful, giving him the chance to go to the Senate because of "the privilege and the honor of being born in the single greatest society in all of human history."

Contact Thomas Fitzgerald at 215-854-2718 or tfitzgerald@phillynews.com, or on Twitter @tomfitzgerald.