By the beginning of 2019, most of the Democratic field had absorbed this new conventional wisdom. The field was racing left, treating the consensus on progressive Twitter as though it were a simulacrum of the real Democratic Party. On some issues, like a higher minimum wage and more generous Social Security benefits, bold left-wing positions really do capture strong political majorities. But on other issues, many candidates have exposed themselves to damaging general election attacks: on reparations for slavery, decriminalizing the border, and giving health-care coverage to undocumented immigrants, among others…

Indeed, two candidates seem to have realized this problem. Kamala Harris and Cory Booker have dropped out of the left-wing arms race and begun positioning themselves as heirs to the Obama style of optimism and uplift. But it’s already getting late, and each candidate has already devoted most of the time between their launch and the Iowa caucuses trying to avoid getting dragged on Twitter. What’s more, the fact that both Harris and Booker are trying to squeeze into the same political space increases the probability that neither will achieve escape velocity…

If Biden isn’t up to this, he needs to collapse soon enough for another mainstream liberal Democrat like Booker or Harris to take his place. Yet it’s at least as easy to imagine he will stay in the race, locking down the party’s center, while Sanders and Warren continue to try to outflank each other on the left. There’s still some time for the race to change. At the moment, the most dire scenario looks disconcertingly real.