The last five national four-way polls: Tie, Clinton by two, Clinton by two, Trump by two, and now a tie again in the new one from the NYT/CBS. One month ago today, Clinton led by 6.3 points in the RCP average. Her lead now is down to 1.1. Debate stakes: High.

If you read the post last night on Trump’s new lead in Ohio and Florida, you can guess one of the big contributing factors to the tightening national race.

In a four-way race, Mr. Trump and Mrs. Clinton are tied at 42 percent each. Gary Johnson, the Libertarian candidate, has the support of 8 percent of likely voters, and the Green Party nominee, Jill Stein, takes 4 percent.

The third-party candidates draw their strongest support from younger voters. Twenty-six percent of voters ages 18 to 29 say they plan to vote for Mr. Johnson, and another 10 percent back Ms. Stein. A little more than one in five political independents say they will vote for one of the third-party candidates.

Yesterday in Quinnipiac’s new national poll, Clinton led Trump 55/34 head to head among the 18-29 crowd. Once Johnson and Stein were added to the mix, her support collapsed to 31 percent. Here, the introduction of the third-party candidates is enough to drive her two-point lead overall in the head-to-head race (46/44) down to a dead heat. She leads Trump big among young adults in the two-way race, 48/29, but Obama won that group 60/37 four years ago. Hillary has a major problem with millennials. Coincidentally, her campaign made an announcement this morning:

https://twitter.com/aseitzwald/status/776421117300908032

If you can’t convince millennials to stick with the Democratic Party with Hillary, maybe you can do it with Bernie Sanders and Elizabeth Warren. Relatedly, there’s another reason buried in the NYT/CBS data to explain why Clinton is struggling:

Thirty-six percent of registered voters overall now say they are very enthusiastic about voting, the same as a month ago. But one disadvantage for Clinton is that enthusiasm among Republican voters has remained steady, while enthusiasm among Democrats has dropped. Among Democratic voters, the percentage that is at least somewhat enthusiastic has dropped from 77 percent in August to 64 percent today, while the percentage of Democrats who are very enthusiastic has dropped nine points – from 47 percent to 38 percent.

More specifically, Trump supporters are more excited about voting than Clinton’s supporters are. Forty-five percent of Trump voters say they are very enthusiastic about voting, compared to 36 percent of Clinton backers who feel that way.

That helps explain why Clinton’s comfortable five-point lead among registered voters falls to a two-point advantage head-to-head once you look at likely voters. Democrats just aren’t as enthusiastic, and it’s hard to explain that in terms of either her “deplorables” remark or her fainting on Sunday — although the attempted cover-up of her pneumonia might be reminding voters of why they don’t trust her, her biggest political problem. (This poll was conducted between September 9-13, meaning that it incorporates some reaction to her rough weekend.) Nate Silver made an interesting point this morning about why Clinton is suddenly struggling. She’s facing a double whammy in which she’s had a bad run in the news lately (Clinton Foundation, “deplorables,” the swoon), which is driving down her numbers, and yet most people are convinced she’ll win in the end and doesn’t need their votes, which is … also driving down her numbers, freeing soft supporters up to vote third-party on the assumption that Trump has no chance. Being a bad candidate whose victory seems assured is a master recipe for driving down enthusiasm.

If Silver’s right, though, then her problem will be self-correcting to some extent. As the media trumpets the polls this week showing Trump surging into the lead in key states, the left will freak out and the word will trickle down to those soft Clinton supporters that she needs their votes after all. You’d expect to see her claw back some millennial support from Johnson and Stein over the next few weeks in that case. Likewise, if Clinton does well at the debates, it’ll end her bad run in the news and start a good one, which will further cushion her numbers. Democrats could even argue that it’s better to have Trump scaring the hell out of their voters now, to shake them out of complacency about the outcome, then it would be to have him trailing the entire race and then suddenly surge big at the end before soft Hillary voters wake up to the threat. That’s probably the best they can do, though, to make the sh*t sandwich of the last few weeks palatable.

As for Trump’s own numbers, he’s out to a 5.9-point lead in today’s LA Times daily tracker, his biggest advantage since late July. The July surge was easily explained, though — it was a bounce after the convention, and convention bounces fade. This new surge is more encouraging for him given the timing, as voters really start to pay attention to the race, and the fact that it’s not a reaction to any one event. One word of caution, though: Although the race has tightened, Trump is still stuck in the low 40s in most four-way national polls and is in the 43-44 range in most (but not all) surveys of the two-way race. There’s improvement there, as his ceiling in the four-way used to be 39-40 percent, but he hasn’t broken out yet. Maybe that’ll happen after the debates. It may already be happening in some battleground states. He hit 47 in Florida and 46 in Ohio in the CNN polls yesterday.