President Bill Clinton is applauded by Al From, chairman of the Democratic Leadership Council, left, and Sen. Joseph Lieberman (D-Conn.) before addressing the council in Washington on Nov. 13, 1995. (Wilfredo Lee/AP)
Jeremy C. Young is an assistant professor of history at Dixie State University, director of the DSU Institute of Politics and Public Affairs and the author of “The Age of Charisma: Leaders, Followers, and Emotions in American Society, 1870-1940.”

How can Democrats beat Donald Trump in 2020? Nancy Pelosi thinks she knows the answer: “Own the center-left, own the mainstream,” she told the New York Times recently, and avoid left-wing “exuberances” like the Green New Deal or Medicare-for-all.

Pelosi’s view — that whoever speaks for the center of the electorate has the best chance of winning the election — isn’t a new one. Political scientists call it the “median voter theorem,” and it’s been popular among Democratic leaders like Bill Clinton since the late 1980s. But other Democrats have believed the exact opposite: that the best presidential candidates are outspoken liberals who draw a clear contrast between their positions and Republican policies. “We cannot win by being Republican-lite,” Howard Dean told enraptured crowds in 2004. “We’ve tried it, it does not work.”

Democrats have been arguing for 30 years about whether centrists or liberals are more electable. What’s interesting, though, is that the answer may actually have changed during that time. In 1987, when Pelosi entered Congress, swing voters decided presidential elections, and centrist policies probably were Democrats’ best chance of getting elected. Twenty-two years later, though, elections are determined by voter turnout, and it’s more important that the party pick a candidate who actually excites Democratic voters.

The median voter theorem became popular among Democrats reeling from the electoral losses of the 1970s and 1980s. With the exception of Southern moderate Jimmy Carter, who was the Democratic nominee in 1976 and 1980, every Democratic presidential candidate of the 1970s and 1980s was a liberal — and every one of them was defeated by crushing margins.

Sen. George McGovern (D-S.D), most famous for his opposition to the Vietnam War, lost to Richard Nixon in 1972 in an unprecedented 23-point drubbing. Former vice president Walter Mondale, who famously promised to raise taxes during the 1984 campaign, lost to Ronald Reagan by 18 points. Most heartbreaking of all, Gov. Michael Dukakis (D-Mass.), up 17 points in the polls in the summer of 1988, succumbed to a barrage of negative campaigning alleging he was soft on crime and weak on the military; he lost to George H.W. Bush that year by eight points.

In the face of such demoralizing defeats, organizations such as the Coalition for a Democratic Majority and the Democratic Leadership Council (DLC) argued that Democrats could win only by moving to the center. Democrats, DLC founder Al From explained in the late 1980s, were losing because of their support for ever more expensive entitlement programs and their retreat from tough-minded foreign policy. To survive, From insisted, the party needed to embrace “mainstream values” and “market-based solutions.”

During the 1990s, From’s centrist approach seemed to have struck electoral gold. Arkansas Gov. Bill Clinton, a former DLC chairman, won the presidency in 1992 and romped to reelection in 1996. As president, Clinton applied adviser Dick Morris’s strategy of “triangulation,” adopting some Republican policies in an effort to win moderate voters. Clinton worked with Republicans in Congress to curtail welfare, deregulate the telecommunications industry and repeal Depression-era restrictions on banks. “The era of big government is over,” he famously declared in 1996.

In the two decades since Clinton left office, however, center-left Democrats such as Al Gore, John Kerry and Hillary Clinton have struggled to replicate the former president’s success at the polls. To be sure, none of these candidates possessed Bill Clinton’s appeal to African Americans or young voters or his skill at retail politics. Still, there’s no denying that the centrism that elected Clinton in the 1990s no longer carries the track record of success it once did. Meanwhile, polls consistently show left-wing firebrand Bernie Sanders leading Trump in a general election matchup, confounding many observers.

So what’s changed? Surprisingly, the answer is the voters themselves. The DLC’s centrist strategy was based on the idea that elections were decided by “swing voters” — moderates whose votes were truly up for grabs between Democrats and Republicans. In 1992, over 10 percent of the electorate voted for a different party’s presidential candidate than they had four years earlier. As political scientist Corwin Smidt has shown, though, the polarization of American politics over the past two decades has dramatically decreased the number of swing voters, even among independents. In 2012, only 5 percent of American voters changed their presidential vote from the previous cycle, the lowest percentage in at least 60 years.

Today, the most crucial question American voters face isn’t whom they should vote for, but whether they should vote at all. In a political system with few swing voters, American elections are now decided primarily by turnout: which candidate can inspire more excitement and bring more supporters to the polls. Barack Obama won the presidency twice on the strength of unprecedented turnout among youth and African American voters. Meanwhile, Trump’s inflammatory rhetoric turned off centrist voters but generated increased turnout among working-class whites in key swing states, winning him a narrow victory.

If the most extreme candidates win today’s elections, does that mean Sanders is the most electable Democratic candidate? Not necessarily. In an election driven by turnout — and one where Trump is guaranteed to fire up the Republican base and to infuriate centrists — the most electable Democrat is the candidate who excites the largest number of voters in the broad Democratic coalition. That could be a moderate, as Pelosi suggests, or a leftist, or someone popular with young voters, minority voters or women. In truth, we don’t yet know which candidate is best positioned to increase Democratic turnout. That’s the point of a lengthy primary cycle: We can observe each candidate on the campaign trail, working to connect with voters, and get some sense of how they’ll do in a general election.

But if we want to find out which Democrat is the most electable in 2020, we can’t fall back on decades-old theories about appealing to the “center-left” or the “mainstream.” We’re just going to have to see what the voters think.