Progressive populists pining for a leftist presidential nominee cannot be pleased with how the primary run-up is proceeding. Many thought that Joe Biden’s early lead was an illusion, propped up by name recognition, and would fizzle once his ideologically checkered past was held up to the light. But after three months in which we have rehashed Biden’s record on the Anita Hill hearing, busing, abortion and two-income families, the former vice president has retained his lead in national and early state polls.

Biden’s current lead is hardly insurmountable. He generally polls in the low 30s nationally. In post-debate state polls, he notched 28% support in Iowa, and just 21% in New Hampshire. If you took the two candidates with the greatest appeal on the left – Sens. Bernie Sanders and Elizabeth Warren – and melded them together, that candidate would tie Biden in Iowa and beat him in New Hampshire.

However, neither Sanders nor Warren has any incentive to step aside for the other. Each polls well enough to justify staying in the race, as almost every national and state poll has them in second or third place. And each has built a huge small-donor base — approximately 746,000 donors for Sanders and 421,000 for Warren, according to the New York Times — which will keep them well-financed throughout the primaries.

But there’s no getting the around the fact that Sanders and Warren are in each other’s way. If Biden’s level of support has a hard floor of around 30% nationally and in Iowa, then neither Sanders nor Warren can afford to splinter the leftist vote, especially because there’s so little either can poach from the second- and third-tier candidates.

Warren, as of this moment, appears to have more opportunity to broaden her base than Sanders, but that’s only of marginal potential benefit. In last week’s national Morning Consult poll, Warren is the top second choice of both Kamala Harris and Pete Buttigieg supporters, who have been in fourth and fifth place, respectively, in most surveys. Harris and Buttigieg are moderates compared to Warren and Sanders. (Harris recently told the New York Times that, unlike Warren and Sanders, “I’m not trying to restructure society.” Buttigieg opposes single-payer health insurance and universal free college.) But in that poll, Harris and Buttigieg combine for only 15% of the vote, and only about one-quarter of those voters would jump to Warren — roughly amounting to about a 3-to-4-percentage-point gain. Plus, Biden is the second choice of about one-fifth of the Harris-Buttigieg voters, meaning if those candidates faded, Warren wouldn’t easily make up much ground.

Moreover, neither Sanders nor Warren could immediately claim the entirety of the other’s support if one did drop out. According to the Morning Consult poll, the second-choice of most Sanders supporters is not Warren, but Biden. About a quarter of Sanders supporters would shift to Warren, presumably because of their ideological proximity. But almost one-third would go to Biden, evidence that ideology is not the only factor driving voter decisions. (For Warren supporters, Sanders is the top second choice, with 25%. But 22% would go to Harris and another 17% would go to Biden.) Demographic appeal and electability concerns are also likely factors.

There are distinct demographic differences in the Sanders and Warren bases working to keep these supporters apart. As Politico recently summarized, “In poll after poll, Sanders appeals to lower-income and less-educated people; Warren beats Sanders among those with postgraduate degrees. Sanders performs better with men, Warren with women. Younger people who vote less frequently are more often in Sanders’ camp; seniors who follow politics closely generally prefer Warren. Sanders also has won over more African Americans than Warren…”

And electability calculations can vary. A Sanders supporter might drift to Biden, thinking he’s the second-best hope for winning over Rust Belt workers. A Warren supporter might view a socialist candidate as political suicide, and opt for Harris or Biden as fallback options.

So overtaking Biden is not as simple as just having either Sanders or Warren step aside. But with the two each taking up the biggest shares of the electorate except for Biden, and if Biden’s current level of support proves durable, it’s hard to see how one can win with the other hanging around.

Perhaps if both remained in the race, one of them could still eke out a victory in New Hampshire, where they are familiar New England neighbors and Biden appears weak. But if neither can stop Biden in Iowa, even though the Democratic Hawkeye electorate is very liberal and very white, Biden would remain viable by the fourth contest in South Carolina, no matter what happens in New Hampshire. And if Biden is a decent shape heading into the first primary state with a heavily African-American electorate, he may not be stoppable.

All of this analysis flows from a presumption that Biden can’t go much lower in the polls than where he sits today, based on the fact that the most damning aspects of his record have already been lobbed at him, to little effect.

Yet we can’t dismiss the possibility Biden could still self-destruct. While he has always been, in his own words, a “gaffe machine,” every new verbal flub gins up questions about his age and his mental acuity. So far, his supporters have been a forgiving bunch. But if Biden suffers an indisputable “senior moment,” his numbers could collapse, throwing open the race and creating space for Warren, Sanders and more. (Sanders is the second choice of 24% of Biden supporters, and for Warren it’s 20%, suggesting that a Biden bust would not automatically give either progressive a leg up, nor one of the more moderate alternatives.)

However, it’s not much of a strategy to wait for Biden to implode, as it puts Biden in control of his own destiny. If progressive populists want to seize control of the primary, and the party, they would be best served by taking action. Since neither Warren nor Sanders can be expected to sacrifice their candidacy to boost the other before Iowa and New Hampshire, the only way for left-wing voters to maximize their influence is by taking the initiative to consolidate around a single candidate.

The members of the Democratic Socialists of America last week tried to put their thumb on the scale, formally declaring the organization would endorse no other Democratic nominee but Sanders, as the only self-proclaimed “democratic socialist” in the presidential race.

The DSA only has 56,000 members, not nearly enough to force the rest of the left to follow its lead. Still, the socialists have the right idea. If the progressive populists of the Democratic Party want to call the tune for the primary, they need to choose.