To fully harness the energy from the demographic revolution, Mr. Sanders will need to strengthen his support among African-American voters who were more resistant to his candidacy when he faced Mrs. Clinton. His strong support among younger African-Americans could help, but he would be best served by choosing as his running mate an African-American with strong electoral appeal, such as Stacey Abrams, the former minority leader of the Georgia House of Representatives who received more African-American votes in a statewide election than anyone not named Barack Obama.

In addition to those particular parts of Mr. Sanders’s strength, he is also well-positioned to win back those voters who defected in 2016 because Mrs. Clinton was too moderate for their tastes. For all the focus on Obama-Trump voters, it was Obama-Stein voters who created the critical cracks in the Democratic firewall (the increase in votes for Jill Stein from 2012 to 2016 was greater than Mr. Trump’s margin of victory in Michigan and Wisconsin). Of all the remaining candidates, Mr. Sanders is the most likely to reclaim those Democratic voters who defected to the Green Party in search of a more progressive standard-bearer.

Much of the angst about Mr. Sanders topping the ticket stems from fear about negative fallout in down-ballot congressional races. Here, too, the concerns are overblown. In the vast majority of congressional districts where Democrats ousted Republican incumbents in 2018, it was enthusiasm and the high turnout of Democratic voters that made the difference, much more than alienated moderate Republicans switching their party allegiance. In all but five of the 41 seats picked up by Democrats, increased Democratic turnout alone would have been enough to flip the seats without any Republican crossovers.

While some small number of down-ballot House races could become more competitive, that risk is offset by the opportunity for Democrats to flip even more seats by mobilizing younger and more diverse voters. In 2018, Democrats fell just 1,000 votes short in both the Seventh District of Georgia, for example, where there is a sizable African-American population, and San Antonio’s 23rd District, which is more than half Latino. There are several other seats where Democrats could make additional gains with Mr. Sanders atop the ticket.

The empirical evidence shows that there is no need for alarm about Mr. Sanders being the Democratic nominee, and even some cause for confidence. If you want to engage in theoretical thought experiments, a useful exercise would be to ask how many people who voted for Hillary Clinton in 2016 would switch their votes to back Mr. Trump just because Mr. Sanders was the nominee? Common sense suggests that the answer is infinitesimally small.

If that is the case, then Mr. Sanders would win the popular vote. As for the roughly 78,000 votes in three states that flipped the Electoral College, the particular strengths that Mr. Sanders brings to the contest strongly suggest that he could close that gap and make the leap into the Oval Office.