Tim Pawlenty is, by all accounts, a capable politician and nice guy. He might help Romney electorally, although the impact of the vice presidential selection is usually over-estimated.
But Romney needs to seriously consider who will be his partner in governance and for all his virtues, Pawlenty does not have any Washington or national security experience. Since Romney himself also doesn't have experience in these areas this is a factor to be considered. The White House is a unique environment and under a far more intense public microscope then any governor's mansion. A President Romney will want an experienced governing partner who can provide high-level advice.I remember a DC-insider telling me that Gore and Clinton were like peas in a pod, and on the surface at least the same would seem to apply to Romney-Pawlenty. I have no information about their personal dynamics - which is critical - but as an experienced politician Pawlenty probably knows how to play nice with the other kids. In the past 40 years, Senators have been the VP and getting along is a key skill. Governors have been less able to adapt to the second-fiddle role of the VP. But Pawlenty's career and behavior show little indication that he would be a loose cannon.
Since 1977, vice presidents have been Senators or experienced DC-hands. The last governors who served as vice presidents were Nelson Rockefeller (for Ford) and Spiro Agnew (for Nixon). The last (and only) time a vice president with no DC experience played a significant policy role as a presidential advisor was Garret Hobart as McKinley's vice president from 1897 to 1900.
Just to reiterate, I have no problem whatsoever with Pawlenty, who seems like a responsible and realistic politician. But Romney needs someone who can tell him about things with which he is unfamiliar - like the Senate.
This highlights Garret Hobart, who as McKinley's vice president played an important role. Hobart had no national political experience, but he had been speaker of New Jersey's House and president of New Jersey's Senate. He ingratiated himself to the US Senate and was generally praised for handling the Senate smoothly. However, the gulf between the Senate of 1896 and the Senate of 2012 is vast. Hobart's experience at the state level probably translated well to Washington.
But no longer, state legislatures are very different animals - for example each US Senator has a 50+ person staff, his or her own tiny bureaucracy. Plus there are the well-staffed committees and agencies attached to Congress (like CBO, CRS, GAO etc.) Also, foreign and military affairs are not areas where Romney is knowledgeable and Pawlenty will have little advice to share. Minnesota does not have a CIA equivalent (as far as anyone knows) and he has not had to manage relationships with enemies and allies.
Returning to Clinton and Gore who enjoyed an excellent relationship, Gore was a Senator and well-regarded on arms control and other national security issues. By all accounts the two men hit it off - but also, when push came to shove Gore had unique areas of expertise he could bring to the table.