Let’s look at where things stand with the Democratic candidates in Iowa as of right now, according to FiveThirtyEight:

1 Sanders: 22.0% 2 Biden 21.5% 3 Buttigieg 15.5% 4 Warren 14.4% 5 Klobuchar 10.2% 6 Yang 3.6% 7 Steyer 3.5%

If you’re a Bernie Sanders supporter, you might be thinking: “OK, we’re in a good position. It’s close but we’re a little bit ahead.”

But even if that number holds, Sanders might actually be crushed in Iowa, thanks to the unusual voting system there and particular effect of candidate No 4. Sanders supporters need to do everything possible to keep people who like both Elizabeth Warren and their candidate from actually voting for Warren.

At an Iowa caucus site, there are two rounds of voting. In the first round, a candidate must get at least 15% to stay in for the second round. If they get less than 15%, their voters are released and can support another candidate.

So in the second round, candidates are hoping to pick up the voters who supported the ones who lost in the first. Delegate totals are based on the numbers from the second round.

Let’s assume for the moment that in one given location, all Pete Buttigieg voters have Joe Biden as their second choice, and all Warren supporters have Sanders as their second choice. I am simplifying, but this will be the trend. And let’s assume that in the first round of voting, the result exactly matches FiveThirtyEight’s polling.

What happens? Well, Sanders, Biden and Buttigieg all break 15% and keep their votes. Warren does not crack 15% so she gets no delegates and her voters are released. Sanders picks up Warren’s 14.4% while Biden picks up nothing. (Let us temporarily forget all the other candidates exist, which is not difficult.)

In the second round, the result is: Sanders 36.4%, Biden 21.5%, Buttigieg 15.5%. Sanders collects the most delegates and comes out of Iowa strong.

But here’s the devastating thing. Because of the structure of the Iowa caucus voting system, a tiny switch could result in a completely opposite outcome.

Let’s say Buttigieg and Warren’s vote totals are swapped. In the first round, Warren gets 15.5% and Buttigieg gets 14%. What happens in the second round? Warren’s votes do not go to Sanders. Instead, Buttigieg’s go to Biden: Result: Biden 35.9%, Sanders 22%, Warren 15.5%.

Headline the next day: BIDEN DEFEATS SANDERS BY 13 POINTS.

Never mind that Sanders was the second choice of all the Warren voters. It doesn’t matter. Joe crushes the caucus, his frontrunner status is restored, people in the press stop talking about the possibility of Sanders winning the presidency, Sanders doesn’t get the Nevada and South Carolina endorsements that would have come out of a strong caucus showing. The campaign limps on, and will struggle to catch up. (The state apportionment of delegates is more complicated than this, but this is how the voting itself will go.)

In fact, it’s even more perilous than that. Even if Warren gets 14% in the first round, if, say, enough Tom Steyer and Amy Klobuchar supporters come over to join her, she can remain viable for the second round, meaning that her voters would not go to Sanders and enough Klobuchar people might go to Biden to help him win even if together Warren and Sanders have a far higher percentage of voters. If, say, Warren got 15%, and Buttigieg’s 14% and Klobuchar’s 10% all went to Biden, Biden would win in a landslide.

Supporting Warren therefore drastically increases the chances that Biden will be the ultimate nominee. This is what happens when progressives “split the vote”. It runs the catastrophic risk of destroying the chances of having a progressive presidency, and handing the nomination to the truly awful Biden.

The unique dynamics of the Iowa caucus may mean that the entire Sanders campaign is severely damaged if Warren breaks 15% and Buttigieg and Klobuchar don’t. Sanders people need to persuade Iowans not to vote for Warren. (And maybe give Buttigieg a little bump, or at least don’t do anything to undermine him. We need him not to drop below 15%.)

Every time progressive organizations treat a vote for Warren and a vote for Sanders as equally furthering the progressive cause, or call for “unity” without rallying behind a single candidate, they make it more likely that progressives will all lose. Everyone volunteering for Warren might as well be working for Biden.

To prevent a Biden nomination, the time to unite is now. Warren had a chance: Sanders amassed more volunteers, got more donations and is doing substantially better in the polls. Now that the actual voting is starting, progressives cannot afford to split their bloc. They need to unify behind Sanders.

In These Times and the Nation have both run articles pushing the narrative that you can support and like both. I am sure many people do. But don’t actually vote for Warren. Divided support is destroying the chances of a progressive presidency.

Let’s remember the stakes: there are good reasons to think Biden would not fare well against Donald Trump. But even if he was elected, nobody seriously believes he can accomplish anything meaningful. He has said as much himself: “Nothing would fundamentally change.” So much for solving climate change.

We cannot afford four more years of Trump. And we have an unprecedented opportunity to throw him out and replace him with something far, far better. How tragic would it be if that opportunity was destroyed because some clung to a losing candidate, dividing the movement and squandering precious votes?

How will history look back on this? Will the Warren campaign’s jeopardizing of this chance have seemed worth it?