Polling expert pinpoints the key indicator that could tell us Trump's fate in the election

With the election less than three weeks away, thousands of Americans have taken advantage of early voting in states ranging from Georgia to Texas to North Carolina. David Wasserman, house editor for the Cook Political Report, discussed this abundance of early voting with NBC News' Andrea Mitchell on Friday — and Wasserman pointed to Sumter County, Florida as a possible way to gauge how the election will ultimately turn out.

"It's true that we're both seeing historic early voting, and it's a drop in the bucket because we're headed for likely 150 million to 160 million votes cast this year — which would be record-shattering," Wasserman told Mitchell.

Sumpter County, Wasserman stressed to Mitchell, is an important place to keep an eye on because it is full of retirees and 65-and-old voters — who could play an important role in determining whether President Donald Trump or former Vice President Joe Biden wins Florida's 29 electoral votes.

"It's going to be difficult, this Election Night, to figure out what's going on in a lot of states because a lot of the upper midwestern states are going to need probably days or perhaps even weeks to be able to count all of those mail-in ballots that lean D," Wasserman explained. "But one thing I'll be watching early on Election Night is Florida. And in Florida, counties are permitted to begin counting ballots 22 days before Election Day, and I'll be watching Sumter County — which is where the bulk of The Villages is."

The Villages, Wasserman noted, is "the largest retirement community in Florida."

Wasserman told Mitchell, "The median age there is 68.9 years old, and the Trump campaign has placed so much importance on The Villages that (Vice President Mike) Pence visited last weekend. Trump is holding a rally tonight in Ocala, which is right next door. And Democrats took delight in these images, last weekend, of a flotilla of 500 golf carts with Biden flags that were on their way to drop off mail-in ballots. That's anecdotal, but what we know from the polls is that Biden has really made inroads with white seniors — particularly in the last couple of weeks."

The Cook Political house editor added that Biden is "ahead, on average, among seniors, 53-44%. Compare that to the final polls in 2016, which had Trump up among seniors 49-44 over Clinton. And so, if there is this gray revolt, it should be apparent early on Election Night because, keep in mind: Sumter, in 2016, 84% of the vote there was cast early. And they reported their entire batch of early votes pretty shortly after the polls closed. We could know by 7:15 PM whether Trump has a massive problem with seniors. And if he can't win Florida, then he can't win a second term."

Watch the clip below: