Richard Nixon, who tied Franklin Roosevelt’s record by running on a national presidential ticket five times, said that he ran to the right to win the Republican nomination but then ran back toward the center in the general election. In the 2004 presidential campaign, however, President George W. Bush and Karl Rove found a new path to electoral victory by turning out their base rather than attracting the undecided.

Now, Democrats face this dilemma in 2020: Should they nominate a candidate who is further to the left in order to energize their base (think Bernie Sanders, Elizabeth Warren, or Kamala Harris), or should they instead steer center-left with Joe Biden?

The decline of the swing voter (someone in the center who might vote either way) has created the opportunity for the ugly, shrill style of campaign that seeks to mobilize a candidate’s base. Bush campaign strategists Karl Rove and Matthew Dowd figured out in 2004 that independent persuadable voters had declined from 22% to only 7% of the electorate, prompting them to spend less time on swing voters and more on turning out their base. That trend has largely continued, with political scientist Corwin Smidt estimating that swing votes have declined from 15% of the electorate to less than 5% over the past four decades. A recent Pew Research Center survey says that while 38% of voters identify as independent, only 7% “decline to lean towards a party.”

Although President Barack Obama had built a strong and diverse base in 2008 and 2012, candidate Hillary Clinton could not hold his coalition, with over 4 million Obama voters not even showing up to vote in 2016. Donald Trump ran a classic base campaign, carefully targeting white voters and nonvoters disillusioned by the economy, immigration, and big government. Indeed, his speech in Orlando, Fla., this week launching his 2020 campaign was more of the same, right down to the old campaign phrases and audience responses.

Where does this center vs. base campaigning leave the Democrats in 2020? All the early energy in the party was from the Left, with socialist Bernie Sanders now more of a mainstream party leader than in 2016. Pushing far left were other strong candidates Elizabeth Warren and Kamala Harris, with still others in the hunt. Based on the mistaken assumption that any Democrat could beat Trump, Democrats were preparing to go hard-left in an effort to mobilize a base of young and ethnic voters.

But not so fast. When the more moderate Joe Biden entered the fray and immediately led in the polls, the party began to rest on the horns of a more difficult dilemma. Would it go center-left with Biden and risk alienating young leftist voters or stay the hard-left course?

For now, it would appear that the far left is at least pushing Biden himself further left, with his recent flip from not favoring repeal of the Hyde Amendment limiting federal funds for abortions to reversing himself a few days later following a liberal outcry. Biden also introduced a huge $1.7 trillion climate change plan since, as Bernie Sanders tweeted, “There is no ‘middle ground’ when it comes to climate policy.” You can bet that more liberal candidates will hold Biden’s feet to the fire on issues such as the Green New Deal, government-funded healthcare, and the like.

Pressure to run hard-left creates challenges for Biden’s campaign. If he goes too far left, he risks losing the center, which might be more in play than usual against Trump. If Biden does not go far enough left, he risks energetic young Democrats staying home or even facing a left-wing third-party challenger in the general election.

A base campaign, centering on turnout more than persuasion, leads to narrow, loud appeals and an ugly year ahead. A campaign of persuasion aimed toward the middle risks another loss for Democrats.

David Davenport is a contributor to the Washington Examiner's Beltway Confidential blog. He is a research fellow at the Hoover Institution. He is the co-author, with Gordon Lloyd, of How Public Policy Became War, published May 7.