If Portman wins in 2010, I would place him among the five most likely senators to be president someday.
...and the most likely senator to be the 2012 GOP VP nominee (IMHO).
Not if the nominee doesn't have foreign policy experience (which is likely at this point, considering the likely spread of candidates).
How do you define "foreign policy experience"? Portman will have served in both houses of Congress and served as US trade rep. Is that less "foreign policy experience" than any other senator who's likely to be considered for VP in 2012? Are you expecting Romney to pick John McCain or Richard Lugar as his running mate?
He was trade representative for about a year, and when he was in the House he was on the Ways and Means and Budget committees. That's a pretty thin foreign policy resume.