Earlier this year, Republicans seemed to have potential opening with members of the large millennial generation, who played a key role in Barack Obama's two victories.
That chance appears to be slipping away, according to the latest Harvard Institute of Politics poll of 18- to 29-year-olds.
Asked which party they would like to see win the next presidential election, millennials favored the Democrats by 20 points, 56%-36%, the survey found. That Democratic advantage is up from a 15-point edge in the spring, noted the poll's director, John Della Volpe.
Compared with the spring poll, Democrats gained ground most notably among the youngest members of the generation, aged 18-24, Della Volpe said.
In 2008, when Obama was first elected, young people were "true outliers," Della Volpe said, giving extremely heavy support to Obama. Since then, young voters have become more polarized between the two parties, much like the rest of the electorate, but the shift toward the Democrats signals a problem for the GOP, he said.
Until this survey, the youngest millennials appeared more open to Republicans than older members of the generation. That gap appears to have disappeared, he said.
The poll, taken before the recent terrorist attacks in San Bernardino, showed majority support for sending U.S. troops to fight on the ground against the Islamic State militants in Iraq and Syria.
But only about 1 in 8 millennials said they would definitely or strongly consider enlisting in the military. About 6 in 10 said they would not.
The survey of 2,011 Americans aged 18-29 was conducted Oct. 30-Nov. 9. It has a margin of error of +/- 2.8 percentage points.