• Turnout will be very high. The 2018 election featured the highest midterm turnout since 1914, the first time U.S. senators were popularly elected. If the historical relationship between midterm and general elections holds, 2020 would bring the highest share of the voting-age population to the polls in half a century, and perhaps since 1908.

Much depends on whether voter mobilization crosses party lines or remains asymmetrical as in 2018, when Democratic turnout was much higher than Republican. On the one hand, President Trump’s presence on the ballot will draw out supporters who stayed at home last year. On the other hand, relative to 2012, African-American participation in 2016 fell while it surged among white working-class voters, which suggests that Democrats have more room for growth.

• Despite the rise of cultural issues, the economy will matter. In every election since 1980 except 1992, an increase in economic growth between the third and fourth year of a president’s term has been followed by victory for his party, while a decrease was followed by defeat. The slowdown of economic growth from 2.9% in 2015 to 1.6% in 2016 roughened Hillary Clinton ’s road to the White House. A predicted slowdown from 2.9% in 2018 to an estimated 2.1% in 2019 and 1.8% to 1.9% in 2020 would create a headwind for President Trump’s reelection campaign.

• President Trump is likely to receive significantly less than 50% of the popular vote, and a smaller share than his Democratic opponent. In the past three general elections, the Republican nominee has averaged 46.3%—almost exactly what Mr. Trump received in 2016—compared with 50.7% for the Democrat. In the past five elections, the Republican average has been 47.5% versus the Democrats’ 49.9%. Since Mr. Trump entered office, his job approval has seldom exceeded the share of the vote he received in 2016.

Mr. Trump prevailed narrowly not because he did better than the average Republican nominee but because Mrs. Clinton underperformed the average Democrat. The missing votes went to third-party and independent candidates, whose total share rose from 1.7% in 2012 to 5.7% in 2016.

Galston, author of “Anti-Pluralism: The Populist Threat to Liberal Democracy” and other works of political analysis, adds that “The 2012 figure was no fluke. In the four elections from 2000 to 2012, the share of the vote not going to the two major parties averaged 2%. The 2016 election was the outlier…” Galston notes further that in 2016, “the Libertarian candidate received nearly 4.5 million votes, about 3.3% of the total cast, including 3.6% in Michigan and Wisconsin and 4.2% in Arizona.”

Libertarians have reason to be displeased with Trump, including his tariff policies, rejection of Libertarian tolerance values and accommodation of rigid evangelical views on reproductive rights. But it is unclear whether Democrats can win an adequate share of their votes in the key states. Glaston notes further that,

• If the popular vote is close, the states that proved decisive in 2016 probably will remain pivotal in 2020. In the Blue Wall triad—Pennsylvania, Michigan and Wisconsin—Mr. Trump’s job approval has been consistently lower than in Florida, Georgia and Texas, where it stands at or above 50%, as it also does in Ohio. Three other states—North Carolina, Iowa and Arizona—occupy an intermediate zone in which Mr. Trump’s popularity is higher than in the Blue Wall but lower than in the South.

In other words, if the president can hold his Democratic challenger’s popular-vote advantage at or near the 2 percentage points of 2016, he may well prevail again in the Electoral College. At the other end of the spectrum, if the Democrat were to approach Barack Obama ’s 7-point margin in 2008, victory over Mr. Trump would be assured. Even if there is a huge mobilization of Democrats in solidly blue states, a 4-point popular-vote advantage would probably include enough voters in swing states to create a blue Electoral College majority. It’s impossible to determine exactly where the tipping point lies between 2 and 4 percentage points.

Galston concludes that “Democrats should have learned from 2016, the outcome of a presidential election is starkly binary, and the cost of defeat is very high. They should choose the candidate who maximizes their chance of winning…this means—first and foremost—the candidate who has the best chance of carrying the states that Mr. Trump pried off the Blue Wall.”

This is the foremost challenge facing Democratic rank and file and each of the presidential candidates when the primary and caucus season begins in five months. In addition to front-runner Biden, there are several other candidates who can make a compelling case that they can win back enough of the rust belt states that are required for an electoral college victory. The time to hone that message and pitch it with gusto has arrived.