When a democratic candidate loses a presidential race, the candidate gets a mark of shame that precludes any serious future run for the presidency. Think Mike Dukakis, Al Gore, and John Kerry. Hell, even Jimmy Carter, who won the Presidency one time, but then became disgraced by in the eyes of the D.C. bobbleheads because he lost the second time.
This does not happen with Republican candidates. When they lose, they instantly graduate into public figures who continue to be taken seriously by the Washington establishment. Think Bob Dole and John McCain. But there is no better example of this phenomenon than Mitt Romney.
By any measure Mitt Romney was a disastrous presidential candidate. He lost his run for the presidency not once, but twice! In fact, the main reason that John McCain got the GOP nomination in 2008 was that he was viewed the best alternative to Mitt Romney. For some reason, that primary loss did not brand a giant L on Mitt's forehead, so he tried again and got the nomination in 2012 only because he was the only clown in the clown car who didn't make the money people nervous. Then he ran against an incumbent president who came into office with high hopes but who was mostly branded as a disappointment by election day. Mitt still lost in the general election, and he lost really badly.
How badly? Put aside the 5 million vote difference and the lopsided 332-206 electoral college tally. Remember how people kept talking about how bad of a candidate Al Gore was by pointing to the fact that he could not even win his home state of Tennessee? Mitt had houses in three different states and he lost all of them (Massachusetts, California and New Hampshire). He also lost Wisconsin, the home state of his Vice President. And he lost Michigan, his boyhood home and the state where his father had served as governor. If Romney were a Democrat, the prospect of a third run for president would be a punchline to a joke.
But Romney is not a Democrat, so bring it on!