Listen carefully and you will hear the sound of politicians evolving, as one Democrat after another runs to align with the gay marriage movement, inspired by fast-moving polls.

"Evolve" is what politicians do when a stance that was once a plus turns into a problem and calls for a slight change of tune.

The usual setting for making these changes is when a pro-life Democrat or pro-choice Republican fixes his eyes upon national office, and a sudden doubt will impinge upon previous certainties.

It is true that sometimes people do change their minds on these issues, but not so predictably, and it also seems odd that these "conversions of conscience" so often go only one way in each of these parties, which is the direction its donors and bases approve.

How this occurs illustrates two things about how politicians view social issues: They evolve when not doing so becomes politically dangerous; and they evolve fairly easily, since most of them really don't care.

How do we know this? It's there in the record, of both parties, going back years.

As a beginner, Obama supported gay marriage; opposed it when he ran for the U.S. Senate; continued opposing it through his first term as president, and evolved only in June of last year, when pushed by Vice President Biden, and by (apparently) Hollywood donors, who seemed about to withhold the big bucks.

Bill Clinton approved Don't Ask, Don't Tell, signed the Defense of Marriage Act in his first term as president, and denounced both only recently, when he no longer had to face voters, and his wife wanted left-wingers' support.

"What are the odds that every Democratic politician with presidential hopes who once expressed pro-life sentiments ... would have epiphanies on abortion that would send them all in the pro-choice direction?" National Review's Rich Lowry asked six years ago.

Lowry noted that both George Bushes had evolved in the other direction, and that Rudy Giuliani and Mitt Romney had each changed directions three times; Romney saying in 1994 that the death of a relative's sister had shocked him into the pro-choice position, while years later (while running for president) a report about stem cells "shocked him back into being pro-life."

Giuliani was pro-life at the start of his first run for mayor, pro-choice at the end of it, and evolved back in the other direction when he began running for president.

In 2007, Lowry said of the former mayor that "he 'hates' abortion -- something he didn't mention when he gave opening remarks at NARAL's 'Champions of Choice' lunch in April 2001."

Does this mean all of these are unprincipled scoundrels? Not quite. It just means that they have their priorities, and in their minds getting to work on the big-ticket issues is worth a cave now and then on the small issues.

And to them, the "values cards" are the small issues: "Almost no major politician really cares about it," Lowry says of abortion, and E.J. Dionne, in a column written months earlier, said pretty much the same thing: "Candidates are rarely willing to say outright what's true for so many. ... [T]hey do not consider [it] the most important issue in politics, and that it is not the reason they entered public life."

What are the reasons? War and peace, the economy, big or small government, the safety net, taxes, the debt. The values issues are "small" but their backers care deeply and act as gatekeepers at the portals of power, where access to big things resides.

So politicians finesse things as long as they can, but when the crunch comes there's only one answer: That's when they start to evolve.

Examiner Columnist Noemie Emery is contributing editor to The Weekly Standard and author of "Great Expectations: The Troubled Lives of Political Families."