In the four years since President Obama swept into office in large part with the support of a vast army of young people, a new corps of men and women have come of voting age with views shaped largely by the recession. And unlike their counterparts in the millennial generation who showed high levels of enthusiasm for Mr. Obama at this point in 2008, the nation’s first-time voters are less enthusiastic about him, are significantly more likely to identify as conservative and cite a growing lack of faith in government in general, according to interviews, experts and recent polls.

Polls show that Americans under 30 are still inclined to support Mr. Obama by a wide margin. But the president may face a particular challenge among voters ages 18 to 24. In that group, his lead over Mitt Romney — 12 points — is about half of what it is among 25- to 29-year-olds, according to an online survey this spring by the Harvard Institute of Politics. And among whites in the younger group, Mr. Obama’s lead vanishes altogether.

Among all 18- to 29-year-olds, the poll found a high level of undecided voters; 30 percent indicated that they had not yet made up their mind. And turnout among this group is expected to be significantly lower than for older voters.

“The concern for Obama, and the opportunity for Romney, is in the 18- to 24-year-olds who don’t have the historical or direct connection to the campaign or the movement of four years ago,” said John Della Volpe, director of polling at the Harvard Institute of Politics. “We’re also seeing that these younger members of this generation are beginning to show some more conservative traits. It doesn’t mean they are Republican. It means Republicans have an opportunity.”

Experts say the impact of the recession and the slow recovery should not be underestimated. The newest potential voters — some 17 million people — have been shaped more by harsh economic times in their formative years than by anything else, and that force does not tend to be galvanizing in a positive way.