Mitt Romney gives Marco Rubio a tryout on the trail

Don't get too excited out there, Marco
  The Mittstakes march on:

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

But as he campaigns across the Keystone State ahead of its Tuesday primary, a different contest is coming into focus ' the race to serve as the former Massachusetts governor's running mate.

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.

This event was scheduled when Rick Santorum was still in the race, but it's notable that Romney decided to keep it on the schedule. Clearly, he's giving Marco Rubio a shot on the trail, just like he did with Paul Ryan in Wisconsin.

Both Ryan and Rubio are on the same page as Mitt Romney on all the major issues, though in Rubio's case the fact that he is Cuban-American has given some Republicans hope that he might be able to help Romney with his difficulty attracting Latino voters. Beyond his ethnicity, however, it's not clear why Rubio would boost Romney's prospects. Sure, he hasn't embraced Romney's fiery rhetoric, such as calling Arizona a model for state immigration policy, but substantively, Rubio supports Arizona's "Paper's Please" law and opposes a path to citizenship through the DREAM Act'just like Mitt Romney.

But if Romney is looking for someone who is telegenic and more charismatic than he is'as well as someone who excites conservatives'Rubio fits the bill. And not just because his name isn't Rob Portman.