In March 2010, Jim Geraghty published what was, to that point, “The Complete List of Obama Statement Expiration Dates.” It listed about 25 or so promises the president broke in his first year in office, plus an addendum of about 20 promises that “expired” during the campaign. In the two years since, there have been more, which Geraghty has documented as well. And the most recent of these has also become the most famous: President Obama’s self-proclaimed “evolution” on the issue of gay marriage.

Unlike his opponent, however, the media has resolutely refused to trifle the president with the appropriate label: the president is quite clearly a “flip-flopper.” Why the double standard? There is more to it than the obvious media bias.

As the Washington Post notes in an interesting article on the subject (please ignore the Post’s unforgivable headline), since John Kerry and, to a lesser extent, Al Gore, were cast as craven opportunists, it is not enough that Romney is a Republican and Obama a Democrat. But those party tags do actually factor into it, the article finds, though not simply because of the visible press bias. The article describes a new study based on an experiment testing voters’ reactions to flip-floppery, in which they are asked to react to one political type who promises to change his positions as the people do, and the other who promises to stay true to his principles:

These candidates represent a classic argument in political philosophy between the view of John Stuart Mill, the British philosopher who said that democratically elected officials should reflect constituents’ views, and that of Edmund Burke, the Irish-born political thinker who argued that we elect representatives with strong values so they will follow their principles.

Voters who preferred Candidate B — Burke’s view — responded much more negatively to candidates who changed their minds on issues, said Barker, director-designate of the Institute for Social Research at California State University at Sacramento. Those voters generally prefer conservative Republicans and are more likely to rely on religious faith to guide their political choices.

Voters who preferred Candidate A — Mill’s view — were much more accepting of candidates who flipped on issues. These voters, mostly drawn to more liberal, Democratic candidates, tend to be more secular and believe that as the people’s views shift, so should their leaders’.

That conservatives were more drawn to a Burkean philosophy on governing isn’t too surprising. Far more interesting is the finding that liberals are much less likely to object to flip-flopping in the first place.

This helps explain why someone like John Kerry–a starkly unlikable figure for whom the label “flip-flopper” seemed particularly apt–could win the Democratic nomination despite all the obvious red flags of his candidacy. It also helps explain why Mitt Romney had such difficulty winning the Republican nomination even though he had a four-year head start and aside from Rick Perry, who possessed a strong record but who stumbled badly in the debates, the path seemed clear for Romney. He struggled not against other strong candidacies but the popular composite candidate known as Not Romney.

It is conservatives, therefore, who branded Romney a flip-flopper long before he had the chance to face John Kerry’s fate of being so labeled during the general election. The right, not nearly so tolerant of unprincipled politicians as the left, immediately flagged what seemed like Romney’s politics of convenience.

The other key takeaway from this is that it surely depends on which issues a candidate flip-flops. The Post article, to emphasize this, begins with Abraham Lincoln’s flip-flop on federal intervention to free slaves. But here is how the Post frames the other element in choosing the “right” issue on which to evolve:

In the end, voters are especially willing to accept a shift in politicians’ positions “if it’s an issue where the public has evolved in its own thinking,” Garin said.

With this in mind, it’s useful to look at two of the candidates’ more controversial changes. Obama’s switch on gay marriage, polls indicate, show him to be swimming with the tide. The public on the whole may not be overwhelmingly in favor of gay marriage, but the trend is toward wider acceptance. It is logical to expect those who have undergone similar “evolutions” on the policy to give the president the benefit of the doubt here.

Romney’s more controversial change, however, is on abortion. It’s true that he has embraced the pro-life position, but voters–especially those on the right–remain skeptical. As such, he may be swimming with the tide–self-identified pro-life voters are increasing, while pro-choice voters are decreasing–but conservative doubt prevents him from fully capitalizing on the switch.