A new Quinnipiac poll of likely voters finds Hillary Clinton up five percentage points in a one-on-one contest with Donald Trump, but her support dwindles significantly in a four-person race, particularly among young adults.

The national survey shows Clinton leading Trump 48 to 43 percent when excluding third-party challengers Gary Johnson and Jill Stein, but the margin between them falls to just 41-39 after expanding the field. Johnson, the Libertarian nominee, takes 13 percent—within shouting distance of the 15 percent national average he must pull for inclusion in the presidential debates—and Stein, the Green party candidate, gets 4 percent.

Johnson and Stein harm Clinton more than Trump, especially with likely voters aged 18 to 34. Among that group, which accounts for 18 percent of the poll's sample, Clinton leads Trump by 21 points in a two-candidate matchup. But she only leads by two points with Johnson and Stein included—and her closest competitor isn't Trump:



Clinton nets 31 percent of under-34 voters, Johnson 29 percent, and Trump 26 percent, with Stein netting 15 percent.

A majority of respondents say they would like to see Johnson admitted as part of the debate field, with independents (66 percent) and, perhaps surprisingly, Democrats (54 percent) the groups most in favor.

The four-way results mirror other polls taken in September. According to the RealClearPolitics average, Clinton's lead over Trump is only 42 to 40 percent, with Johnson and Stein netting 12 percent combined.

The poll results can be found here.