McCain please pick Mitt Romney as your VP. As a democrat I could not think of a more perfect choice for you (other than Jeb Bush). The contrast between the tickets could not get any more striking. Romney is the anti-Biden and so many levels: authenticity, experience, and likability.
First of all, Romney is a serial flip flopper. He's flip flopped on abortion, he's flip flopped on immigration, he's flip flopped on timetables in Iraq, and he's apparently flip flopped on John McCain. Romney and McCain hated each other, remember McCain and Huckabee colluding to screw Romney in West Virginia. McCain's having a little fun today playing a clip of Biden attacking Obama, but that's nothing compared to the wealth of material Romney has provided on McCain. Romney accused McCain of not being a true conservative, and McCain said of Romney that "he has consistently taken both sides of every major issue." Putting Romney on the ticket would serve to cement the ticket's status as the double talk express.
Secondly, Romney doesn't know a damn thing about foreign policy. The closest thing he has to international experience is working on the Salt Lake City Winter Olympics. Biden would absolutely crush Romney in the debates. And on the stump Romney would be able to offer little more on foreign policy than praise for the surge. He'd end up responding to every foreign policy question with "I'll have my staff get back to you" more than McCain does on domestic issues. Romney's experience in general is thin, and based mostly on his work in the private sector which translated into a mediocre governorship of Massachusetts.
Thirdly, while Biden comes across as relatable due to his Irish catholic Scranton roots, Romney's Mormonism still gives voters pause. Romney will have to spend time explaining his religion and how he differs from the polygamist sects that are currently in the news. And while rational people won't hold Romney's religion against him, evangelicals aren't known for their rationality. They are skeptical of McCain's commitment to their causes as it.
Lastly, Biden comes across as authentic due to his "shoot from the hip" colloquial way speaking, Romney comes across as extremely calculated. His statements, appearance, and policies appear to be focus tested and subject to change with public opinion. Romney comes across as a politician first, second, and third. This difference is furthered by the large gap in wealth between the Biden and Romney. Biden's wealth (as it is) comes from his senate salary and his wife's teaching salary. Romney, by contrast, is worth hundreds of millions. Romney and McCain own a dozen homes between them (although you can be sure Romney knows exactly how many properties he possesses). Romney reinforces the McCain campaign reality of being out of touch with working folks.
While Romney may help McCain some in the West which is where Obama wanted to make new inroads, I imagine Romney would damage McCain's chances everywhere else, in the South, in Florida, and in the Northwest. So please McCain, pick Mitt Romney.