Hey it's a hot new political trend! If you watch the polls closely and see public opinion trending steadily in a particular direction -- say, a direction leading away from a position you once embraced in order to get elected -- just go along with the public opinion trend and say your own views are "evolving."
For example: once upon a time a politician really could not survive a general election unless they said they didn't agree with gay marriage. For many Democrats, this presented a conundrum. Quite a few of their supporters were in favor of gay marriage, but a whole lot more weren't, and in order to capture moderate votes they had to throw gays under the bus. So these politicians ended up just muttering something about "Nothing against the gays, but my religion won't let me." Voters shrugged and pulled the lever and voila, one of these folks -- Bill Clinton -- got elected president. Twice!
Now, several states have legalized gay marriage, and its fiercest opponents are either dying of old age or have quit responding to polls. Mainstream politicians still have to oppose it to be taken seriously as cultural moderates, but they don't have to oppose it quite as vehemently as they used to. And who knows, maybe in ten years they'll be able to proudly officiate at the weddings of all their gay friends who they've been quietly selling out for decades.
Exhibit A: former president Bill Clinton, father of "don't ask, don't tell" and historical gay marriage opponent, can now safely say that his views on gay marriage are "evolving."
In the early part of this decade, political figures tended to shun anything that looked like waffling. Now "evolution" is in. But just as "consistency" threatened to become a code word for "pig-headed resistance to facts," "evolution" is on its way to becoming "the craven public embrace of once-derided political views, now that they're not so unpopular."