That is well below the 78 percent of registered voters overall who intend to take part in the election. Because young voters are more likely to support President Obama, that could be bad news for Democrats this year.
Voters in the 18-29 age group historically are less likely to turn out than older people, but in 2004 and 2008 the deficit was smaller, Gallup said.
Hispanic voters this year support President Obama by a 2-1 margin. But they also appear less likely to vote, with only 64 percent saying they will definitely cast ballots this year.
Black voters, a group that overwhelmingly supports Obama, are less enthusiastic about turning out this year than they were in 2008. But 76 percent said they plan to vote.
Polls give the president, at best, a slight lead over his likely Republican challenger, Mitt Romney.
"The challenge for Obama is that many of his strongest support groups, including young adults, blacks and Hispanics, have historically turned out to vote at lower rates than other subgroups," Jeffrey Jones, a Gallup analyst, said. "So maintaining a relatively high level of turnout among these groups is a key to Obama's winning a second term."
The data is based on Gallup Daily Tracking poll May 1 to July 10, with a random sample of 30,952 registered voters, including 2,803 young voters, 2,422 blacks and 1,622 Hispanics. The margin of error is 1 percentage point for the entire sample, 2 points for young and black voters and 3 points for Hispanics.