There’s a wake-up call buried in a new national poll from NBC/Wall Street Journal out today for Democrats: Marco Rubio ties Hillary Clinton among young voters, 45-45.

It’s worth saying off the bat that polls conducted this early in the election cycle, especially ones that sample national audiences, are not predictive of the eventual victor come November. But since Democrats are gambling their future on their natural affinity with young voters, who are more diverse and liberal than past generations, they ought to be paying attention to this finding now.

Rubio aside, Clinton has been struggling to attract the young voters she would need to assemble the winning coalitions that got President Obama to the White House twice — coalitions that relied crucially upon turning out young voters. She is losing young women in particular to Bernie Sanders in the primary, a dynamic that the New York Times recently chronicled.

So if and when Clinton becomes the Democratic nominee, she’ll need to win back many of these disaffected voters. Young voters are traditionally the demographic that’s least motivated to vote, and their apathy could spell doom for her campaign.

Add to that a youthful, attractive Republican nominee, whose immigrant background and forward-looking rhetoric naturally appeals to younger Americans. Notably, it’s Rubio who poses the real threat: The poll has Trump trailing Clinton 54-33 with the same group, while Clinton handily wins over Ted Cruz 49-40.

This is not by chance. While Cruz has made evangelical voters the main target of his campaign, Rubio has shaped his candidacy around the need to make inroads with traditionally Democratic voting blocs.

But again, this is just one poll. Its sample of 1,000 voters is relatively small and represents a national audience, which tells us little since presidential elections are decided state by state, rather than through a national popular vote.

Rubio also hasn’t yet received the all-consuming scrutiny that would certainly accompany a primary win. It’s possible that these young voters aren’t acquainted with many of Rubio’s social positions, notably his strong opposition to same-sex marriage (which he says isn’t "settled law" and abortion (which, he has said, he would oppose even in cases of rape or incest. These put Rubio strongly at odds with young voters, who are much more socially liberal than the electorate as a whole.