“Unity” is something both parties want to achieve: Everyone focused on the same goal and delivering the same message as they put aside their differences in service to a larger cause. But it can be elusive. Right now, after a long period in which Republicans were the ones fighting amongst themselves while Democrats stood in formation against President Trump, those roles may be reversing.

That’s because despite years of bickering between the far right and the GOP “establishment,” all Republicans now want the same things: Protect Trump at all costs. Get him reelected. Prevent “socialism,” a.k.a. a liberal policy agenda.

On the other side things are more complicated, as Democrats go through what every party does when it has a contested presidential party. The candidates disagree with each other — which almost inevitably causes anger and bitterness among their supporters — and the party has to decide what it stands for and what it means to be a Democrat.

The person at the center of the unity question is Sen. Bernie Sanders (I-Vt.), but first, let’s take a quick tour around the landscape. On Tuesday, Sen. Kirsten Gillibrand (D-N.Y.) became the first presidential candidate to wade into a primary campaign when she endorsed a challenger to Rep. Daniel Lipinski (D-Ill.). No Democratic congressman is more reviled by liberals than Lipinski: He opposes abortion rights and marriage equality, and voted against the Affordable Care Act.

Lipinski has retained the support of House Speaker Nancy Pelosi (D-Calif.), who has a blanket policy of opposing primary challenges to sitting members. And the Democratic Congressional Campaign Committee recently announced that it would blacklist any political consultant who works for a candidate challenging an incumbent Democrat. The blacklist has been attacked by members including Rep. Alexandria Ocasio-Cortez (D-N.Y.), who argue that vigorous primary debates make the party healthier.

In the middle of this debate is Pelosi, who used to be seen as a far-out lefty but is now often heard arguing for less ambitious policy goals (she is dismissive of Medicare for all) and noting that a big reason Democrats won the House in 2018 was because a lot of moderates won in swing districts, and the party shouldn’t jeopardize those seats.

She’s right that many moderate Democrats won last year, because that’s what happens when you have a wave election. You pick up seats in close districts, and the candidates who win in them will usually be more moderate. So two things happened simultaneously: House Democrats gained a group of moderate members, and also a group of extremely progressive members. Tension almost becomes inevitable, and it isn’t surprising that Pelosi is more concerned about the moderates, who are at greater risk of losing next year.

But if you look at the things that Ocasio-Cortez says when she talks about this conflict, you’ll notice that she frames the disagreements as substantive (which policies are the best) and strategic (what will be most effective for the party). What she doesn’t do is argue that the Democratic Party is a fundamentally corrupt institution that must be overcome if true change is to be achieved.

Here’s where we get to Sanders, who has been making that argument his entire career, including now, during his second run to be the party’s presidential nominee. It’s why he still refuses to officially join the party, calling himself an independent.

But now Sanders finds himself in what could be an awkward position. He built his political persona on being an outsider, an insurgent, the one who argues that Democrats are not going far enough or being progressive enough. That’s who he was at the start of his political career four decades ago, that’s who he has always been and that’s who he will always be. There is no set of policy positions the party could adopt that would make Sanders say, “The Democratic Party is now correct,” because it’s just not in his political DNA; Democrats will always be too centrist for him, and fighting them will always be how he defines his project.

The trouble is that he’s the front-runner for the Democratic nomination, which makes it harder to be an insurgent. That’s why Sanders is only too excited to get in a kerfuffle with with the Center for American Progress, the most prominent Democratic think tank, or to see articles like this one, about how establishment Democrats are trying to figure out a way to stop him from getting the nomination. Whatever they come up with will not only be ham-handed and ineffective, it will only reinforce his ability to present himself as a threat to the establishment.

If you’re worried about the bitterness of 2016 repeating itself in 2020 if Sanders doesn’t become the nominee, you should be. Sanders voters will once again be harder to bring into the fold than those of other losing candidates, because a big part of the reason they (at least many of them) joined his campaign is because they believe that the party is corrupt and that any outcome other than their candidate winning the nomination is proof of that corruption and necessarily illegitimate.

Of course, Sanders could become the nominee. But as devoted as his followers are, they make up only a fraction of the Democratic electorate (he gets 20 to 25 percent in most polls). So it’s hard to know whether the party would be more unified behind him or someone else.

And looming in the background is Trump. If the need to defeat him in the general election can’t get Democrats united once the primaries are over and they have a nominee — no matter who it is — then they’ve got real problems.