Ramesh Ponnuru makes this point in his piece "Heartbreak Awaits Republicans Who Love Gingrich":
We already know the basic strategy of the Obama campaign. It will be to portray the Republican nominee as a dangerous right-wing extremist. Romney’s demeanor -- his steadiness, his reasonableness -- would undercut that strategy. It seems likely to be much more successful against Gingrich. After all, it already was: In 1996, Clinton ran against Gingrich as much as he ran against his nominal opponent, Bob Dole. Clinton portrayed Gingrich as callous and radical, and used Gingrich’s ill- considered words, such as his claim that Republican plans would cause the Medicare bureaucracy to “wither on the vine,” against him.(emphasis mine)
Obviously, Ponnuru isn't saying anything we don't already know: The liberal media will use that attack against us. He's also right that that attack won't work so well against Romney- but then, how many of us actually want Romney as President? I certainly don't.
Whomever gets the GOP nomination- assuming it isn't Romney- will need a running mate who can do two things: a) Pull the "middle"- independents and moderate Dems- to our side, since we need more than half the country to vote for our guy; and b) nullify the Democrat "heartless" myth.
Enter Gary Johnson: Two-term Republican governor of New Mexico, a state where Democrats outnumber Republicans two-to-one. He didn't win election and reelection by compromising, either: Johnson dramatically cut state government regulations on businesses, balanced New Mexico's budget every year, and got concealed carry for New Mexico residents. These are three things one wouldn't expect to happen in a two-thirds Democrat state, but Johnson accomplished them.
The secret to Johnson's success in that state is that he has an excellent capacity for educating: He uses facts, presented in an inoffensive way, to change minds on critical subjects. He has a gift of reason and casual "good guy" appeal which is attractive to the center and immunize him from the Democrat attack machine.
I think our eventual candidate would do well to capitalize on Gary Johnson.