The midterm elections may have been a sign of what's ahead for the 2020 presidential election: experts say the voter turnout could be the highest in a century.

The big picture: According to Michael McDonald, an elections expert at the University of Florida, turnout for the 2020 presidential election could be as high as 67% — the highest it's been since at least 1916. If that happens, President Trump will have a tougher fight for a second term.

He's driving turnout among those most unhappy with him (younger voters and people of color) even when he's not on the ballot.

among those most unhappy with him (younger voters and people of color) even when he's not on the ballot. And Trump voters aren't a growing demographic group. The share of whites with less than a 4-year degree — Trump's constituency — dropped by 3% from 2014 to 2018.

Between the lines: McDonald is basing his prediction of "a hundred-year storm" on the 2018 midterms, which had the highest off-year election turnout in more than a century (50%). He says that momentum will only get stronger.

Prior to 2018, turnout rates for midterms since 1974 had been languishing around 40% to 41%.

"It doesn't seem to be the candidates who were running in 2018; there wasn't hyper competition driving turnout; some states made it easier to vote, but that can't explain it because the increase was happening in every state," McDonald said. "So the only explanation is Donald Trump because he’s the only major factor that’s changed in our politics since 2014."

Turnout in the 2016 presidential election was 60%.

"The safest prediction in politics is for a giant turnout in 2020," said Larry Sabato of the University of Virginia. "Nobody's going to believe the polls after 2016, and everyone will assume a tight race."

"Anti-Trump Democrats and the pro-Trump base will both set human adrenaline records — the intensity across the country is going to be spectacular. Let's hope the polling places can accommodate the crowds."

With that kind of increase, turnout rates would likely go up for everybody. But:

Older white people already tend to vote at high rates, and they're close to their maximum turnout already.

already tend to vote at high rates, and they're close to their maximum turnout already. By contrast, you'll see bigger turnout increases among young people, people of color, and low-income people — generally important constituencies for the Democratic Party — because they vote in lower numbers.

you'll see bigger turnout increases among young people, people of color, and low-income people — generally important constituencies for the Democratic Party — because they vote in lower numbers. From 2014 to 2018, turnout among whites with a 4-year degree went up 17%, while non-white voters' turnout increased by 15%.

Yes, but: There could be some increase among Trump voters, too. Turnout among whites without a 4-year degree went up by 12% in the same period. "I'd imagine these people are going to be activated in 2020 with Trump on the ballot," McDonald said.

While the country is becoming more diverse, it's also getting older, particularly in places like Ohio and throughout the Midwest, according to the 2020 Census Bureau projections.

That's another group that Trump tends to perform well with with, especially older white voters.

The Trump campaign isn't buying it. "Predicting turnout this far in advance of an election is a fool's errand," said campaign spokeswoman Kayleigh McEnany, noting that there's no sure way to know which voters will turn out.

The bottom line: For all of his struggles in the polls, Trump is the incumbent and he has a booming economy. But a historic election turnout could wipe out those advantages — and the early signs suggest that's exactly what we're about to get.