WASHINGTON — Andy Welsh of Portland, Ore., was so enthusiastic about President Obama’s first run for the White House that he voted absentee while spending a semester abroad in Spain. But Mr. Welsh, now 26 and an aspiring diplomat, said the gridlock in Washington has been “a real bummer ever since.”

Katie Hermann, 29, a corporate strategist in Chicago, also supported Mr. Obama for president, but when she thinks about the state of politics in America today, she said, “I feel slightly more discouraged than I did back when President Obama was first elected.”

David Durgin, 28, who is putting himself through the University of Colorado and owns a car detailing shop near Denver, did not vote for Mr. Obama and has also soured on Washington. “There’s too much fighting going on between the parties,” he said.

These three voters — Mr. Welsh, a registered independent; Ms. Hermann, a Democrat; and Mr. Durgin, a Republican — reflect what political analysts see as a troubling trend: the idealism of youth is slipping away, replaced by mistrust and a growing partisan divide among voters under 30. These so-called millennials, who turned out in droves to elect Mr. Obama in 2008, are increasingly turned off by politics. Experts fear their cynicism may become permanent.