A couple of weeks back, I asked whether it was possible that Donald Trump might lose millennial voters to a third-party candidate, Libertarian nominee Gary Johnson.

Well, a new poll shows him losing young voters not just to Johnson, but also to Green Party nominee Jill Stein.

The McClatchy poll shows Trump pulling just 1 in 10 votes — 9 percent — among Americans under 30 years old. Hillary Clinton is at 41 percent, while Johnson is at 23 percent and Stein is at 16 percent. Trump is basically tied with "undecided," which is at 8 percent.

It's important to note here that, as with other subsamples of young voters, the poll did not test a large number of them — only 15 percent of the 983 registered voters that Marist College polled for McClatchy, or about 150. That means there is a very high margin of error.

But it comes on the heels of plenty of other evidence that Trump is struggling — mightily — with the youngest American voters.

A Fox News poll this week didn't include Stein, but it showed Trump and Johnson close among voters under 35 — the generally accepted definition of the millennial generation. Trump was at 23 percent, while Johnson was at 19 percent.

In early July, a Pew study that included a larger sample — and larger subsample of voters under 30 — also showed Trump and Johnson virtually tied among teens and 20-somethings.

So the complete picture that's taking shape is: Young people really don't like Trump. His favorable rating among voters under 30 in the McClatchy poll is just 13 percent, with 82 percent unfavorable (!). He does worse with them than he does among Latinos (25 percent favorable, 73 percent unfavorable) and nearly as poorly as he does among liberals (10 favorable, 88 percent unfavorable).

But that also doesn't necessarily mean he'll get just 9 percent or even just 20 percent of their votes. Third-party candidates tend to poll better before Election Day than they actually perform on Election Day. As The Washington Post's Stu Rothenberg wrote Thursday:

If history is any guide (and it has not always been one this election cycle), support for Johnson and Stein will ebb over the next three months.
Supposedly serious Independent or third party nominees like John Anderson (1980) and Ross Perot (1992) saw their poll numbers slide in the final months of those campaigns as voters decided to cast their votes for someone who could win the White House. That same dynamic could well occur again (Perot’s numbers in 1996 and Nader’s in 2000 remained more stable.)
The Greens probably will attract a handful of Bernie Sanders supporters, but Stein’s (and her party’s) agenda is way too far to the left to attract mainstream supporters who otherwise would support Clinton.
And while Johnson’s decision to add Weld to his ticket surely reflects his effort to move the Libertarians to the political mainstream, that party spends as much time arguing internally about what it stands for as it does wooing general election voters.
As long as the Libertarians and Greens seem irrelevant and outside the political mainstream to the average voter, they will not be in the electoral conversation.

Young voters have long been one of the very best demographics for third-party candidates, and Johnson and Stein will spend plenty of time appealing to them.

They might not beat Trump with this demographic, but they can surely do some real damage to his prospects of winning. And for that, Trump has himself to blame.