First, I ignore most single polls. It's what a wide range of polls show that's important. And what a single poll shows over a long period of time.
Second, there is really only one issue that can explain the "enthusiasm gap" closing, and that's the Kavanaugh nomination. The poll was taken in mid-fury, in the middle of a crisis. But that fury is a single-focused matter which will pass. Either he'll be confirmed, or he won't be confirmed. On the other hand, Democratic enthusiasm has been building for two years and is entrenched.
So, let's look at that "single focus."
One possibility (and likelihood) is that Brett Kavanaugh will be confirmed. If so, then Republicans will be very happy, and their "anger" at how badly they think he's been treated will almost-entirely pass. They got what they wanted. Nothing to be enthused about anymore.
The other possibility is that he could be defeated. And while less likely, it's still very possible. But much of that Republican anger will be directed at Republicans. How they are the majority party, yet couldn't get their own nominee confirmed. How it was Republican senators voting "No" that kept him from being confirmed. Some anger, of course, will definitely be directed at Democrats who supposedly abused the confirmation process (as opposed to abused women, but I digress...) -- but they hate Democrats already. This "enthusiasm" is about getting Kavanaugh confirmed.
Further, if Kavanaugh gets voted down, it's not like the process is over, and now there will be a liberal nominee next. Republicans know that Trump is actually still president and gets to name the next nominee, who will be just as conservative. Yes, it's important to them to retain control of the Senate so the next nominee gets confirmed. But that's always been the case, even before Kavanaugh. It was always important to Republicans to hold on to the senate at the mid-terms for any Supreme Court openings that might occur over the next two years. Yet there wasn't any special enthusiasm previously. But this current "enthusiasm" is specifically about Brett Kavanaugh. And it's focused and limited,
On the other hand, if Kavanaugh does get confirmed, while Republicans will be overjoyed and dancing in the streets, their anger dissipated -- Democrats will have their heads on the verge of exploding. Their fury will be profound. The Get Out the Vote efforts will increase significantly.
Moreover, even if Kavanaugh doesn't get confirmed, that won't change things much for Democrats. They'll be thrilled and relieved -- but -- they'll still be furious at the process, how Republicans blocked releasing documents, refused to call witnesses to the hearings, wouldn't question Dr. Ford themselves but hid behind their "female assistant," rushed the process, limited the FBI investigation and tried to push through a sexual abuser and perjurer for a lifetime appointment to the Supreme Court. Additionally, they'll know that Trump does indeed still have the next nomination, and it will be someone just as conservative, so it's critical to take control of the Senate.
And one other matter. Even if enthusiasm is equal and ends up equal (something, as I've noted, I don't believe is and will be the case), the voter bases are not. There not only are more registered Democrats than Republicans, but Independents have been shifting towards voting Democratic this year. That's been the huge problem with Trump's "rallies" -- he's been trying to shore up the base, when he should be trying to expand it. Republicans can't win with the current GOP base, it's too small. Whether they can do so in the next five weeks remains to be seen. I think it's deeply unlikely.
The one thing that favors Republicans is that they tend to vote more in mid-term elections than Democrats. But I think this year, that bet is off the table. For starters, we've seen the results in special elections for the past year. But also, as I've just noted, enthusiasm among Democratic voters should be much greater than among Republicans by November 6 -- AND, just as importantly, Democrats have been organizing rallies and marches for the past two years, and they are far-better positioned for getting out the vote than Republicans.
All that said, it's important to pay attention to enthusiasm polls, and no one should take this election with the slightest complacency. But I truly don't think that's a risk, not only after 2016 electing Trump, but also after Trump's disastrous Helsinki press conference with Putin, taking children from their parents, ending the Iraq Nuclear Treaty, pulling out of the Paris Accord, Muslim travel bans, "sh*thole countries," and...and nominating Brett Kavanaugh -- and so much more.
And if any Democrats ever even come close to losing focus and somehow being complacent and not paying attention, there will always be those daily tweets that bring them back to reality and on full alert.