If you do not want Joe Biden to get the Democratic presidential nomination, we need to do everything possible to make sure people do not vote for Elizabeth Warren in Iowa. As of this writing, the Iowa caucuses are 48 hours away. By failing to unify behind Sanders, the progressive left is walking into a buzzsaw that will destroy us. To see why, have a look at where things stand right now in the Iowa polls as tracked by FiveThirtyEight:

Sanders is leading Biden by a hair, though they’re pretty much completely even. Warren is in fourth place, as she is in New Hampshire, a position reflected in her poor fundraising. Warren is not going to win. But her candidacy can still destroy Bernie. In fact, that’s all it can do.

Notice from the current polls that even a small shift of Warren voters to Sanders would be extremely helpful in opening up a solid lead over Biden. If there is not a progressive victory in Iowa, it will be because there were still people voting for Warren. But things are actually far more perilous than that, because of the weird structure of the Iowa caucuses. There are two rounds and if a candidate doesn’t get 15 percent in the first, their voters can switch to a second candidate.

Let’s consider a precinct in which the first vote broke down exactly according to the current FiveThirtyEight polling. (This is an individual precinct, not the ultimate statewide result.) Warren would only get 14.4 percent. Since Sanders is the most common second choice of Warren voters, he would pick up her votes in Round 2. (For the purposes of explanation, let’s just assume he got all of her votes.) Buttigieg, however, would keep his 15.5 percent for the 2nd round. This situation is good for Bernie: the centrist vote is split between Buttigieg and Biden, while he gets the progressive vote. But what if there is just a slight difference? Buttigieg drops a point and Warren goes up a point? Then it’s a calamity.

In the second round, Warren keeps her voters. Buttigieg’s voters are released and mostly go to Biden. Biden also gets Klobuchar’s voters. So Biden resoundingly crushes Bernie in this precinct, because the progressive vote was split but the centrist vote ended up united. Thus every precinct where Warren cracks 15 percent in is horrible news for Bernie. If you vote for Warren, you are voting for Biden. Period. If you’re wearing a Warren shirt, look closely at it: It really says “Biden.” Every time a progressive votes for Warren, Joe Biden’s smile widens another inch:

Now, note another thing: We actually don’t want to see precincts where Pete Buttigieg drops below 15 percent! Many of us on the left have attacked him, but at this particular moment he happens to be useful to us. We do not want his support to fall further than it already has. So be nice to the Mayor Pete supporters. Give them a lift to the polls. Do the dance with them. Right now he’s siphoning Biden votes. Leave him be.

This needs to be made clear to every person considering supporting Warren. If their second choice is Sanders, they need to vote for Sanders. Worryingly, even Sanders supporters seem to think it’s fine for Warren to take away potential votes and cause him to lose. Dave Weigel recently asked what Sanders’ voters preferred order of results was. A disturbing number of Sanders people want Warren to do well:

The correct answer, if you actually want Bernie Sanders to be the nominee, is B. Those who choose C are not thinking about how boosting Elizabeth Warren is actually going to play out. Her candidacy is destroying Sanders’ chances of finishing off Biden. It could wreck Sanders completely. She needs to come in fourth place in Iowa, get under 15 percent of the vote, then drop out. Anything else will make things much, much harder.

Some people think that it doesn’t matter how the vote is apportioned between Sanders and Warren, as long as together, they do well. This is delusional. If Joe Biden comes out of Iowa ahead, he gains potentially unstoppable “momentum.” What’s “momentum?” Momentum means: The news media stops running stories about how Bernie might win, and starts talking about Joe’s reclaiming of frontrunner status. BIDEN DEFIES EXPECTATIONS, REBOUNDS TO RESOUNDINGLY DEFEAT SANDERS is the headline. People start endorsing him who had hesitated waiting to see if he’d turn out a dud. Probably including more labor unions (as well as the law firms helping companies destroy those unions, who actually realize which side Biden is on).

Now, you might think: Okay, so Joe will technically win all of the primaries, as Elizabeth Warren hangs on and splits the progressive vote. But Warren and Sanders together will have more delegates. As I have noted before though, there is absolutely no reason to think Warren would support Sanders over Biden, and there is considerable reason to believe that she would in fact end up with Biden. She didn’t endorse Sanders in 2016. She tried to torpedo his campaign this time with a damaging and implausible allegation. She’s held back on going after Biden too hard. If Joe is the clear frontrunner, and her politics are “halfway” between Bernie and Biden, why would she not go with Biden? Certainly she can extract a lot of concessions from Biden. Possibly the VP slot. And she’ll be under immense pressure from prominent members of the party (possibly including Barack Obama himself) not to go with Bernie.

So if Joe wins Iowa, it’s bad for Bernie. And the No. 1 thing keeping Bernie from pulling way ahead is that Elizabeth Warren is continuing to peel off people who would otherwise support him. This does make it extremely frustrating that “progressive” magazines like In These Times and the Nation have promoted the idea we can support “both” and just divide our votes. In fact, doing that will kill the chances of a progressive presidency.

We cannot afford Joe Biden. He’s truly terrible. And all a Warren vote can do at this point is have the same effect as a Florida Nader vote in 2000. The “progressive primary” has been run, and she lost. We gave her a chance. Sanders’ campaign has more support, more money, more organizing capacity, more enthusiasm. If things had come out the other way, this conversation might be different. And I am not critiquing Warren on her political merits here, even though I have done so before. But now that voting is about to start, we can no longer consider splitting. We have to rally behind Sanders.

If you are a Warren supporter, the question you need to ask is: Is the difference between Warren and Sanders so great in Warren’s favor that it’s worth throwing away our shot at a robust progressive agenda? On climate change? On healthcare? If Warren’s doomed candidacy takes Sanders down with it, how will you feel? How will history look back at this election? It will be very clear that we bungled it. We blew our one shot.

We have a very short amount of time. The Iowa caucuses are on Monday. We have got to get everyone behind Sanders. Plead with people: They can like Warren, but they absolutely should not vote for her.