But it still feels strange to see these two men at the top of the pack for a party that has changed so much in the past decade. After all, it's been 15 years since the Democrats nominated a white man for president, and they've done quite well without one. Barack Obama won two presidential elections, and while Clinton lost in 2016, she pulled in three million more votes than Trump.

And in the years since the 2016 election, the party's base has been more than energised: it has been electrified, sweeping off-year elections in 2017, when it replaced more than a dozen white male Republicans in the Virginia House with a new class of Democrats, primarily women, including the first Latina delegates, the first Asian-American delegates, the first openly transgender delegate, and the first openly lesbian delegate. The Democrats also swept the 2018 midterm elections, sending a record number of women to Congress as well as a number of "firsts", including the first Native American and Muslim women to serve in national elected office.

Former vice-president Joe Biden speaks during a campaign stop at a Teamsters union hall in Pittsburgh. Credit:AP

In light of that history, "electable" seems to mean something new. Yet Democrats heading into 2020 seem focused on white male candidates. Not just Sanders and Biden - Pete Buttigieg, the mayor of South Bend, is the only candidate massively outperforming his name recognition, and it seems no coincidence that he, too, is white and male (though unlike Sanders and Biden, he is gay and very young - just 37, making this the first presidential election in which he's eligible to run).

So how to make sense of what feels like a reversion to an earlier understanding of electability? The term does not exist in a vacuum. In a race against an incumbent, electability begins to take on very specific characteristics. In 2004, for instance, Vermont Senator Howard Dean was an early favourite. He had a lot of the hallmarks that Democrats wanted in a candidate: pathbreaking policy ideas for healthcare reform, economic policies aimed at addressing inequality. He also had strong support from the base, using innovative internet strategies to raise money and build a following in an age before such techniques became commonplace.