Mitt Romney seems the most aristocratic. If he's not richer than God, he's apparently richer than the current president and vice president put together, and that's saying something. Nor is he self-made; he began with a base of inherited wealth and inherited political celebrity. His father was not merely a corporate titan, but a political heavyweight who himself ran for president. Mitt's willingness to spend tens of millions from his own coffers to advance his presidential ambitions would have worried the founders. No one in America in 1787 had money on Mitt's scale. Only a few European aristocrats had this kind of spending power, and we have seen what the framers thought of them.
Also, Romney is the only non-senator of the bunch. He is neither a war hero (like McCain, or JFK before him), nor a diplomat (as Hillary Clinton was, de facto, as first lady), nor someone (like Obama) who has spent many years living in parts of the world that Americans know least and need to understand much better.
The best constitutional argument for Romney is one that he has never made: To make amends for America's long history of discrimination against Mormons, voters should consider engaging in electoral affirmative action for Latter-Day Saints.
As an admiral's son born in the Panama Canal Zone, John McCain is fully eligible. (A citizen from birth, he would have been eligible even had he been born on undeniably foreign soil—say, Brazil.) McCain understands foreign threats firsthand, but his "residence" abroad in a Vietnamese POW camp gives him a more combative view of the world than, say, Obama, who also spent years living abroad. (Think of the difference between the Marine Corps and the Peace Corps.) McCain instinctively understands the need to "preserve, protect, and defend" the homeland; whether he understands how to protect the Constitution is iffier. He's the only top candidate without extensive legal training, and his ballyhooed campaign finance "reform" threatened the core First Amendment freedom of criticizing incumbents at election time.
Dynasties come in different sizes and shapes, and the Clinton dynasty is both like and unlike the dynasties that worried the framers. On the one hand, Hillary Clinton's résumé is inextricably intertwined with Bill's. When she boasts far more "experience" than Obama, she doesn't really mean her seniority over him in the Senate. Rather, she means her time as a top political adviser to her husband and unofficial Minister Without Portfolio in his White House. This service makes it hard to say how much she should be made to answer for (or to take credit for) what he did and didn't do.
On the other hand, both Bill and Hillary are largely self-made and not (yet) fabulously wealthy. (Their net worth so far is only tens rather than hundreds of millions.) Their dynasty thus far is intragenerational, not intergenerational. Theirs is a relationship of choice, not blood. At the founding, George Washington in effect adopted Alexander Hamilton as his helpmate. Hamilton was a "favorite son" of sorts—but he was chosen for, not born into, this post. Hillary is like Hamilton in some ways—a gifted lawyer, a brilliant policy analyst, a sharp partisan, and a towering lightning rod.
She is also like Eleanor Roosevelt, who, along with FDR, offered America its first two-for-one presidential package. Eleanor's dramatic redefinition of the role of first lady as a politically significant position is best seen through our constitutional prism. Once the Constitution guaranteed women the vote, new electoral strategies became possible. A male president could appeal to largely male moderates while his first lady could exploit an emerging gender gap and target her own message to largely female liberals. Hillary's candidacy takes the strategy one step further as the ultimate fulfillment of the 19th Amendment guaranteeing women's suffrage.
In turn, Barack Obama's candidacy marks the fulfillment of the 15th Amendment guaranteeing black suffrage. Whereas Romney is the most aristocratic candidate, Obama is the least. Not because he is biracial, but because he is self-made. His parents had little wealth, prestige, and power to pass on, and both are now dead. He has chosen his own faith as an adult (Protestant Christianity, as it happens), and he remains far and away the least wealthy of the Big Four. His children are still young. In these and other ways, the historical figure he calls most to mind is not MLK or JFK or RFK—as so many have suggested—but Abraham Lincoln.
Oh, and as some of the framers would have appreciated, he can give a pretty good speech, too.