That's the theory Noah Millman puts forth in The American Conservative, and recent history seems to back him up.
After comparing Mitt Romney's selection of Rep. Paul Ryan as his running-mate to Bob Dole's selection of Jack Kemp (Daniel Larison notes the differences), Noah Millman makes an interesting observation: Republicans tend to pick vice-presidential candidates who please their base, while Democrats tend to balance their ticket with someone who'll appeal to centrist voters.
Since Reagan, every VP choice by a Republican nominee has been, in some fashion, a base-pleasing move to the right. Bush Sr. picked Dan Quayle to reassure movement conservatives that he would keep faith with the Reagan religion and that they would have a voice in the White House through which to reach him if he didn't. Dole picked supply-side hero Kemp. Bush Jr. picked Cheney, largely to provide heft and ballast to the ticket, but also because Cheney had a rock-solid conservative record in Congress. And McCain picked Sarah Palin, who, at the time of the pick, was ideologically undefined, but who was expected to excite conservatives because of her background and life story.
Interestingly, the Democrats also have a history over the same period of picking running mates perceived to be to their right, or at least from the rightward flank of their party. Dukakis picked Bentsen. Clinton picked Gore - darling of the DLC set. Gore picked Lieberman. Kerry picked Edwards (who, though he ran to the left in 2008, was perceived as a relatively more conservative Democrat in 2004). And Obama picked Biden, who was certainly perceived as being to his right on foreign policy.
Jonathan Haidt is always telling us that conservatives value loyalty more than liberals. Are there other political forces at work here too? Why do the VP picks tend to differ by party? As Millman says, "The last time the Republicans picked a VP candidate specifically intended to moderate the conservative image of the nominee was 1980. The last time the Democrats picked a VP candidate specifically intended to balance their ticket in a leftward direction was 1976."
So the trend has held for quite a while.
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