Around this time four years ago, before the presidential primaries had begun, the most plausible Republican candidates seemed to be reading from more or less the same script. There were differences, to be sure, between Ted Cruz, Marco Rubio, and Jeb Bush, but for the most part, they offered a mixture of social conservatism, budgetary austerity, and neoconservative foreign policy. Even as the field dwindled, Cruz, Rubio, and the supposedly moderate John Kasich—the last mainstream candidates left standing—all supported slashing Social Security and Medicare to make room for large income tax reductions. They were cut from recognizable GOP cloth, if tailored to slightly different tastes.

Donald Trump, meanwhile, was different, and not just in the color of his hair and the length of his ties. While all the other Republicans were converging around the policy positions of Paul Ryan, Trump identified a section of potential GOP voters who were being overlooked. It was to them that he directed his startlingly new positions on trade, immigration, foreign policy, and entitlements; for them that he promised to protect Medicare and Social Security; and for them, that he proposed a noninterventionist, what’s-in-it-for-us foreign policy, and pledged to end free trade agreements.

The majority of GOP voters—as much as 60 percent—didn’t particularly like these positions. (And GOP funders, especially those in the Koch network, saw his policy positions as an outright repudiation of their core ideological commitments.) The ordinary Republican candidates—the 16 not named Donald Trump—knew as much. But in fighting for the “normal” 60 percent of the Republican electorate, they ensured their own defeat. In Illinois, the three conventional Republicans (Cruz, Rubio, and Kasich) took 59 percent of the vote, but because it was split three ways, not one was able to top Trump’s 39 percent. The same thing happened in North Carolina, where voters gave the orthodox candidates 58 percent, and Trump took the state with 40. The strategy may not have been intentional, but it turned out to be foolproof: Carve out a distinct political ideology that appeals to a solid minority of primary voters, and let the rest of the candidates vie for, and consequently split, the rest of the vote.

More than a dozen candidates may run for the Democratic nomination in 2020: governors from the Plains states, senators from the coasts, billionaire entrepreneurs. But the most serious so far—Kamala Harris, Elizabeth Warren, Cory Booker, and Bernie Sanders—run the risk of falling into the same trap as the main Republicans did in 2015. All of them—even the previously ideologically flexible Cory Booker—are competing for the same section of the primary electorate, one that wants to trade in centrist triangulation for social democratic economics. Given the repeated failures of deregulation, fiscal conservatism, and crony capitalism, this is an understandable instinct. Any one of these candidates could win the nomination if he or she were the only one in the mix. But there are (at least) four or five of them, all clustered around the same positions; come next summer, they will be fighting for the same voters, and as a result, they could all lose. It’s the same bad math that afflicted Cruz, Kasich, and Rubio four years ago, only now it’s on the other side.

A recent survey by More in Common, a bipartisan think tank, identified a section of voters it called “progressive activists.” These people account for a disproportionate percentage of voters in Democratic primaries. But they are, More in Common found, just 8 percent of the American electorate as a whole. In other words, many more potential primary voters may be out there who would be open to a different kind of ideological mix than the one offered by the major Democratic candidates. And because no one is fighting for (and splitting) their share of the vote, they could end up deciding the Democratic primary.