WASHINGTON — A majority of 18- to 29-year-olds – a constituency crucial to the success of President Obama’s health overhaul — disapprove of the law, and fewer than a third of those who are uninsured are likely to sign up for coverage, according to a new poll by Harvard University’s Institute of Politics.

The survey, released Wednesday, also found a stark drop in Mr. Obama’s approval ratings among those so-called millennial voters, who have long been his most ardent supporters.

Just 41 percent now approve of the president, down from 52 percent in October 2012, a finding that puts young voters more in keeping with the general population. The younger half of the cohort, those 18 to 24, tilts increasingly conservative; the poll found that 52 percent of this group — many of them too young to have voted for Mr. Obama when he first ran in 2008 — would vote to recall him from office if they could.

“For the better part of the last four or five years, young people have been the outliers,” John Della Volpe, the institute’s polling director, said during a conference call with reporters. He attributed the drop to high expectations, “not just for the president but for Washington and adults in general, that have been unmet.”

The findings bode ill not just for the president but also for Congressional Democrats, who must run for re-election in 2014 facing questions about the Affordable Care Act. They also present problems for the health law itself.

Enrolling young people, who tend to be healthier and need less medical care, is critical to keeping premiums down. But just 29 percent of those who are uninsured said they would sign up for coverage if eligible to do so. (The law allows young people to stay on their parents’ plans until age 26.)

Trey Grayson, the institute’s director, said the views of young people, who have grown up expecting websites to work flawlessly, may be colored by the troubled roll-out of HealthCare.gov, the online insurance exchange. And Mr. Grayson said young people might also be disenchanted with the notion that their participation is required to keep costs down for their elders.

Whatever the reason, the poll found “there are very few aspects of the health initiative” that young voters approve of, Mr. Della Volpe said. Attitudes were relatively unchanged regardless of whether it law was referred to as Obamacare or by its proper name, the Affordable Care Act.

A solid majority, 56 percent, disapproved of the law when it was called the Affordable Care Act. Just 17 percent said the measure would improve the quality of health care; 78 percent said quality would either stay the same or get worse. Half said the law would increase costs, while 46 percent said costs would decrease or stay the same.

The survey, of 2,089 18- to 29-year-olds, was conducted online by GfK between Oct. 30 and Nov. 11 by and has a margin of sampling error of plus or minus two percentage points. It is the latest in a series of two dozen polls that Harvard has conducted of young voters since 2000.