WASHINGTON — Young voters support President Obama by a narrower margin than they did in the 2008 election and backers of his Republican challenger, Mitt Romney, are more enthusiastic about going to the polls, according to a poll released Wednesday.

In a survey by Harvard University’s Institute of Politics, likely voters under 30 favor Obama over Romney 55 percent to 36 percent. Exit polls of voters in the 2008 election showed Obama defeating Senator John McCain, Republican of Arizona, among those 18 to 29 years of age by 34 percentage points, a key to the president’s victory.