PITTSBURGH — Mitt Romney may no longer face any serious opposition on his way to the GOP presidential nod.

Sen. Marco Rubio (R-Fla.) campaigns with Mitt Romney in Pennnsylvania Monday afternoon. (AP Photo/Haraz N. Ghanbari)

This afternoon, Sen. Marco Rubio (R-Fla.) becomes the first potential VP pick to campaign alongside Romney since former senator Rick Santorum (R-Pa.) bowed out of the race earlier this month. Romney and Rubio will appear at a town hall in Aston, Pa., just outside Philadelphia at 1 p.m. Eastern time.

On Sunday, Rubio told CNN’s Candy Crowley that he intends to “respect [the] process” of Romney’s choosing a running mate and that he doesn’t intend to discuss the veepstakes anymore. (Of course, that doesn't mean the VP speculation is likely to die down anytime soon.)

The joint event comes on the heels of an appearance by Romney at a forum of Hispanic business leaders in Arizona, at which some participants argued in favor of the presumptive GOP nominee’s stance on illegal immigration, while others contended that he should soften his position on legislation such as the DREAM Act.

It also comes two days before the topic of illegal immigration takes the spotlight in another venue — on Wednesday, the Supreme Court will hear oral arguments on Arizona’s controversial immigration law.

While illegal immigration may not be as hot-button an issue in Pennsylvania as it may be in border states, it’s worth keeping an eye out for whether the topic comes up at this afternoon’s Romney-Rubio town hall.

Rubio, one of two Hispanic Americans currently serving in the Senate, has been working on a GOP version of the DREAM Act. Romney has said he is opposed to the original version of the DREAM Act but has not yet weighed in on the proposal Rubio is crafting.

Also on Monday, Romney will hold a morning event near Pittsburgh with former Pennsylvania governor Tom Ridge (R), who served as President George W. Bush’s first secretary of homeland security. In the evening, Ann Romney will address the Connecticut Republican Party’s Prescott Bush Awards Dinner in Stamford.