In two weeks the Democratic presidential field will gather in Atlanta for its next debate. As the requirements for inclusion ratchet up and the time until the Iowa caucuses shrinks, candidates will sharpen their appeals – and their attacks.

The pair to watch, Sens. Bernie Sanders and Elizabeth Warren, share many traits. Both are New England-based progressives, outspoken opponents of centralized corporate power and wealth, and candidates who sincerely believe government can and should play a larger role in the lives of Americans.

In the parlance of politics, they occupy the same “lane.” Sooner than later, their paths must cross, and when they do, the results will be decisive. Recent surveys show their collective share of support between 30%-40%. In a field this large, that is an enormous tranche of voters collected around two similarly situated candidates.

How they go about differentiating themselves will rely less on policy and more on politics and personality.

Bernie Sanders has been through this before. Four years ago, he ran an unexpectedly strong campaign and nearly knocked off a titan of American politics. As this year comes to a close, Sanders’s crowds are growing, his polling has stabilized, and he’s moving from a message based in anger to one of unity and cooperation.

Elizabeth Warren is a dynamic campaigner with a highly professional operation. Voters appear to be rewarding her for it. Her crowds, too, have swelled, and her supporters appear as fervent as those of any candidate in the field.

Where the separation will occur is around the tent pole for both their campaigns: Medicare for All. A Bernie brainchild, M4A has the support of Warren and most of the progressive wing of the Democratic Party. Sanders, however, is the rare progressive who openly admits that it and his other programs will cause taxes to go up – for nearly everyone.

Warren, however, has been unwilling to admit what so many voters, her opponents, the media, and health care experts know to be true: Medicare for All will cost an enormous amount and there aren’t nearly enough billionaires around to pay for it all. For Sanders, Warren’s fuzzy math might be his opening gambit against his New England neighbor. But will he stop there? Will Bernie’s operation be able to resist the temptation to remind voters that Warren was a Republican until she was nearly 50? Will he dredge up the controversy around her exceedingly dubious claim of Native American heritage?

For her part, does Warren have what it takes to lay Bernie out? Will she remind all those progressives about how Sanders’ socialism didn’t preclude his Vermont-centric defense of the Second Amendment? Or that, until about five years ago, he wouldn’t even claim to be a Democrat? Can Warren stand on the stage and accuse him of turning his back on his own unionized campaign workers or call him out for the “Bernie Bro” culture so out of step within the “#MeToo” movement?

It’s not clear that when the break occurs, and it will occur, that support will transfer to one or the other. Bernie Sanders supporters are not unlike those of Donald Trump: They’re ferociously loyal and harbor a strong “burn it down” ethos. Warren appeals to the intellectual wing of the party, ready for a woman to retake the nomination and carry her message all the way to the White House. But are they willing to line up for selfies with Bernie? It doesn’t seem likely.

If these disaffected voters stay home as the primaries progress, it could solidify Joe Biden’s lock on delegates and support. Or Mayor Pete Buttigieg can take their place and claim the title of “Electable Progressive.”

We’ve reached the “politics ain’t beanbag” portion of the campaign. Neither Warren nor Sanders can win with the other standing in the way. One or both of them will have to look within themselves and ask if they have what it takes to do what is necessary to win.

We’re still three months away from Iowans pulling into snowy church parking lots to caucus for their favorites. But the match race between Sanders and Warren will play out much sooner than that. How it does may determine not just the course of the primary, but the general election as well.