At first glance, President Trump's reelection chances don't look good. Stories about impeachment and presidential misbehavior dominate the news. Trump's disapproval rating is high. Independent voters are against him. GOP congressmen are retiring from suburban districts that trend Democratic. The generic ballot is about where it was last cycle. Trump's win in 2016, when some 78,000 voters in three states gave him the Electoral College, was a close-run thing. Seems hard to repeat.

And yet liberals are filled with apprehension. They are coming to recognize the potential size of the president's pool of supporters. They fret over the capacities and liabilities of the eventual Democratic nominee. And their concerns are related: Trump's ability to recapitulate or expand his winning coalition depends in large part on the identity of his opponent. Given these uncertainties, it would be foolish to predict Trump's fate. He might even be stronger than he appears.

"Despite demographic trends that continue to favor the Democrats, and despite Trump's unpopularity among wide swathes of the electorate, it will still be difficult for the Democrats to prevail against an incumbent president who has presided over a growing, low-unemployment economy and retains strong loyalty among key sectors of the electorate," write Ruy Teixeira and John Halpin of the Center for American Progress. No conservatives they.

The Democratic difficulty has a name: the Electoral College. Twice in the twenty-first century, the level of the presidential vote has mattered less than its distribution. Trump's people are spread much more evenly across the country than his opponents are. His base of white voters without college degrees, say Teixeira and Halpin, "make up more than half of all eligible voters in critical Electoral College states he won in 2016—including Wisconsin, Michigan, and Pennsylvania—and in key target states for 2020 such as New Hampshire."

Non-college white voters comprised the largest part of the electorate in 2016. Trump won them 63 percent to 31 percent. That margin more than compensated for his 7-point loss among whites with college degrees. Teixeira and Halpin predict that the number of white voters without college degrees will drop next year. But they also recognize that Trump can still win. "If he increased his support across states among these voters by 10 margin points, he would in fact carry the popular vote, albeit by just 1 percentage point."

White voters without college degrees are the reserve army of the GOP. They are a falling percentage of the U.S. population but a rising percentage of the Republican Party. Recently the Wall Street Journal observed that the same number of voters identify as Republican today as in 2012. It is the composition of the party that has changed. In 2010, half of Republicans were white voters without bachelor's degrees. Today 59 percent are. In 2010, 40 percent of Republicans were white voters with bachelors. That number has fallen to under 30 percent.

The movement of whites without college degrees into the GOP is decades old. But the trend accelerated during the Obama years. Why? During his presidency, especially his second term, the regional, religious, and cultural differences between whites of varying educational attainment became more acute.

The Republican Party of 2019 is more rural, more un-credentialed, and more supportive of government intervention in the economy than it was before. And it backs President Trump. His approval among Republicans in the Journal / NBC News poll is 84 percent. None of his erstwhile primary challengers earn more than 2 percent support.

The voters who put Trump over the top have not abandoned him. "Some 62 percent of voters approve of Mr. Trump's job performance in the 450 counties in which Mr. Trump outperformed 2012 GOP presidential nominee Mitt Romney by 20 points or more," according to the Journal. "That is up from a job approval rating of 43 percent in those counties during Mr. Trump's first year in office." It is also 17 points greater than his overall job approval rating in last month's Journal / NBC News poll. If Trump's demeanor and brand of national populism has repelled the educated professionals who inhabit America's economic, political, and cultural institutions, it has failed to drive away his core supporters. It may even attract them.

"Democrats clearly need a strategy that both mobilizes the strong and growing anti-Trump Democratic base and reaches out to voters the party lost to Trump in 2016," conclude Teixeira and Halpin. "Democrats don't need all of these voters but rather just enough of them—particularly white non-college women—to halt the erosion and cut their margins to levels where relatively strong base turnout and new support from white college graduates can offset Trump's advantages in the most important battleground states."

Which is why the identity of the Democratic nominee matters. A study by the Public Religion Research Institute in coordination with Brookings found that one third of Americans say their vote will depend on the winner of the Democratic primary. These uncommitted voters identify as moderate and politically independent. They don't like Trump, but may wind up in his column if the Democratic nominee strikes them as immoderate. A Democratic nominee whose agenda is out of step with the public would be exactly what Trump needs to increase his support among his base and reclaim lost ground among independents and white voters with college degrees.

As I write, Elizabeth Warren leads in both Iowa and New Hampshire.

Now you know why Democrats are worried.