All along, it has been highly probable that Romney will pick someone ultra-safe, scandal free, and puts a zero dent in negative media coverage. Pawlenty has always been safer than Portman. TPaw has already been vetted publicly and by the media with his short-lived presidential campaign, so there will be no shock or surprise factor from the media or voters.
Pawlenty has decent government experience as a mildly popular governor of a liberal state. He has no DC experience, which may or may not be a good thing depending on how you view it. Romney may want to go completely "DC Outsider" for his campaign theme.
It's been obvious for a while the Romney Campaign would view DC experience as a negative and the board was deluding themselves with Burr, Thune stuff. Portman's a fairly safe pick but has more DC time than Pawlenty. If internal polling convinces Romney Campaign Portman would help him in Ohio, he might become the favorite but so far no sign he would. Rubio is riskier and with the economy stalling out, Romney will probably play it safer. Pawlenty's the VP frontrunner.
Ironically, even though Pawlenty actually ran for president himself, his biggest advantage is how transparent McDonnell's and Christie's presidential ambitions are. (McDonnell's bigger problem is women's issues.) Pawlenty was always half running for VP, which was transparent in his retreat from his Obamneycare attack in the debate.
