This primary season has wrecked the idea that momentum lasts more than a few milliseconds, and given life to another grand theory of politics: Demographics are destiny.

Buttigieg and Klobuchar, whose appeal had always skewed to white voters, exceeded their national polling averages in the very white states of Iowa and New Hampshire. Sanders, who has a high floor of support among young people, low-income voters, and Latinos, cleaned up in Nevada, which is almost one-third Latino. And Biden, whose support is highest among black voters and the elderly, dominated in South Carolina, which has the country’s fifth-highest share of black voters.

The demographic-destiny theory has some grounding in political science. A 2019 paper on the “momentum myth” from researchers at Vanderbilt University found that the characteristics of each state’s electorate, rather than some national force of momentum, explained the state-by-state outcome of the 2016 Democratic primary. Across the country, Sanders won where voters were whiter and younger, and Hillary Clinton won in states that were older, with larger black populations.

The Vanderbilt paper found that primary victories don’t change many minds, but they do open donor wallets. And more money means more chances to pick up delegates. Nate Silver, who has criticized reporters for obsessing over polling momentum, has still built an election-forecasting model that rewards candidates after early caucus and primary wins because, as he’s written, early state wins typically lead to more donations and a higher likelihood that rivals will drop out.

If the Democratic nomination process comes down to Joe Biden versus Bernie Sanders, it will be a showdown between the only two candidates who built a steady and sizable base: Sanders with young voters (especially Latinos), and Biden with old voters (especially African Americans). This is an age when the parties are broken, primary voting is fractured among many viable candidates, and the party establishment is too weak to clear the lane early for its preferred nominee. These sorts of contests aren’t chiefly about momentum, or early acceleration. They’re about building a base that’s real, resilient, and ready to vote.