A few months from now, there is a real chance that Sen. Elizabeth Warren (D-Mass.) will be the Democratic nominee for president. There is also a real chance that Sen. Bernie Sanders (I-Vt.) will be the nominee.

One thing is certain: It cannot be both of them.

So it shouldn't be surprising that the two candidates — longtime friends and political allies, we've been told — have started taking shots at each other now that the Iowa caucuses are just a few weeks away. There is a more-in-sadness-than-in-anger quality to the fight that is now bubbling up, but the fight was inevitable nonetheless. Elections are winner-take-all affairs. The losers go home. After a year of campaigning and debating, neither Sanders nor Warren want to go home. In our system, you can't do politics without dabbling in conflict and contrasts.

Let's back up and explain how we got to this point. The New York Times and CNN both reported, using anonymous sources, on a 2018 meeting in which Sanders purportedly told Warren that a woman cannot win the 2020 presidential election against President Trump. Sanders denied the allegation — calling it "ludicrous" — but Warren confirmed the story.

"I have no interest in discussing this private meeting any further because Bernie and I have far more in common than our differences on punditry," Warren said.

It probably isn't a coincidence that the Times and CNN stories appeared about a day after Politico reported the Sanders campaign was distributing anti-Warren talking points to campaign staffers, instructing them to tell Democratic primary voters that Warren has little chance of appealing to swing voters in a campaign against Trump.

Sanders offered a non-denial denial. "We have hundreds of employees. Elizabeth Warren has hundreds of employees," he told the Times. "And people sometimes say things that they shouldn't."

Why shouldn't they?

It's true that plenty of Democrats are nervous about the prospect of open disagreement between Sanders and Warren — worried that a battle between the two will divide the party or give Trump ammunition to use during the general election campaign next fall. "You both are progressive champs & our movement needs to see you working together to defeat your corporate Dem opponents — not attack each other," Democracy for America tweeted on Monday, pleading for unity. "Progressives will win in 2020, but only if we don't let the corporate wing or Trump divide us." In other words, progressive Democrats want unity, now.

But that's impossible. The voting hasn't started yet. For Sanders to win, Warren will have to lose — and vice versa. It's possible that both of them might lose to former Vice President Joe Biden. No matter what scenario you conjure, though, the end result is the same: There can be only one winner.

To win elections, politicians like Sanders and Warren must prove to voters that they are better than the other candidates, even if those candidates are otherwise friends. That message inherently means the other candidates are worse. Sometimes campaigns, especially in primaries, leave that contrast implicit — and Sanders and Warren have gone to great lengths until now to avoid criticizing each other — but usually the "I'm-better-they're-worse" message is made explicit indeed. The process is divisive.

That process doesn't automatically give Republicans a win over Democrats. Sanders gave Hillary Clinton a bruising battle for the Democratic nomination in 2016, but she still won the popular vote against Trump. Clinton, in turn, offered hard criticisms of Barack Obama during the 2008 primaries — and he won the general election rather easily. So Democratic fears about the Sanders-Warren divide are probably overblown.

Indeed, what those recent campaigns prove is that unity is possible in primary campaigns, but usually only after a victor has been crowned and the other candidates — and their supporters — have accepted their losses and are ready to be team players again. Sanders campaigned for Clinton in 2016; Clinton became Obama's secretary of state.

Most polls rank Sanders and Warren among the top three Democratic candidates. Each is vying to become the progressive choice for voters who reject Biden's moderation. So this battle was coming, sooner or later. Don't worry, Democrats — once the dust has settled, everybody can be friends again, and allies in the effort to remove Trump from office. Until then, though, it's time to do politics.

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