The good news for Democrats? Trump is downright toxic in "Global Metros," the largest component of the electorate.

In 2016, Hillary Clinton took 62 percent of all major-party votes cast in these mega-cities and their suburbs, exceeding Barack Obama's shares in 2008 and 2012, as well as her husband's showings in 1992 and 1996. These urban/suburban melting pots are where Trump's rhetoric about immigrants and exhortations for nonwhite congresswomen to "go back" to other countries backfire. Of the 43 House districts Democrats wrested from GOP control in 2018, 28 were wholly or partially located in these metro areas.

Additionally, Democrats are holding their ground in "International Metros." In 2016, Hillary Clinton took 53 percent in these second-tier metropolises, roughly the same share Obama took in 2012 and Bill Clinton won in 1992.

The bad news for Democrats? They are backsliding in "Regional Metros" — the smaller cities and suburbs where residents would have to connect through a larger hub airport to travel internationally, and where Trump's hard line on immigration and protectionism is likelier to resonate. In 2016, Trump took 55 percent of the vote in these metros, up from Mitt Romney's 53 percent in 2012 and John McCain's 51 percent in 2008. And Trump clobbered Clinton in nonmetro counties, taking 67 percent, up from Romney's 60 percent.

But the even bigger problem for Democrats: The "Global Metros" where Trump's unpopularity continues to deepen are disproportionately concentrated in states that aren't poised to be decisive in the Electoral College in 2020.

For example, even though residents of "Global Metros" make up 41 percent of the vote nationally, they make up 76 percent of the vote in California, 66 percent in Illinois, 97 percent in Maryland, 64 percent in Massachusetts, 89 percent in New Jersey and 61 percent in New York — all states that Democrats win comfortably anyway. They're also 50 percent of the vote in Texas, a state where Democrats are on the rise but Trump still won by 9 points in 2016.

But residents of the "Global Metros" are just 44 percent of the vote in Michigan, 34 percent of the vote in North Carolina, 34 percent in Pennsylvania, and 5 percent in Wisconsin — all states that could be near the tipping point of the Electoral College.

Meanwhile, "Regional Metros" and nonmetro areas — the places where Trump achieved liftoff in 2016 — are just 45 percent of the electorate nationally, but they're significantly over-represented in the most critical battlegrounds: They make up 56 percent of the vote in Michigan, 59 percent in North Carolina, 46 percent in Pennsylvania and 68 percent in Wisconsin.

Of the five states the Cook Political Report rates as 2020 toss-ups, Arizona is the only state where residents of "Global Metros" make up a majority, 66 percent — a statistic inflated by the fact Phoenix's Maricopa County is home to a majority of the state's population.

Democrats have declined most sharply in rural America, but it's "Regional Metros" that should concern the party most in 2020.

Not only do these smaller cities and suburbs make up an outsize share of the vote in key states — compared with both rural and "Global Metro" areas — but Democrats still have plenty of room to fall from Clinton's 45 percent share in 2016. If Democrats can maintain altitude in the "Regional Metros," their gains since 2016 in "Global Metros" should be enough to overtake Trump and reoccupy Air Force One. If they can't, Trump could very well win re-election while losing the popular vote again.

For now, Democratic presidential primary candidates are drawing enthusiastic crowds to rallies in places like New York, Seattle, Austin and San Francisco. But to beat Trump, Democrats will need to ask themselves which candidates' proposals will fly in Erie, Saginaw and Green Bay.

Methodology: This analysis reviewed major airline flight schedules, FAA data on primary commercial airports and Census definitions of Metropolitan Statistical Areas (MSAs). Metro areas without a primary commercial airports were categorized based on the nearest primary commercial airport. For example, the Ann Arbor, Michigan, metro area is served by Detroit's airport and categorized as "global." In addition, all vote shares reflect percentages of the two-party vote only.

Sources: U.S. Census Bureau and Federal Aviation Administration