So we've made it through Labor Day weekend, and for Democrats that means the primary contest enters a new phase and it’s time to get serious about picking a winner.

In any other presidential cycle, Democrats would have the luxury to go with their heart when they vote in a primary. Democratic voters often flirt with the “heart” candidate before going with their head, but circumstances this time might prevent even that typical dalliance. An element of calculation has kept Joe Biden well ahead of his nearest rival, Elizabeth Warren, based on the assumption and borne out by polls that he would have the best chance to beat President Trump.

When the candidates assemble in Houston for their third debate on September 12, frontrunners Biden and Warren will face each other on stage for the first time. Viewers can take the measure of these two very different candidates, while Bernie Sanders tries to reclaim his primacy over a surging Warren, and the lower-polling candidates—Amy Klobuchar, Cory Booker, Julian Castro, Beto O’Rourke—attempt to break through.

The severe winnowing, sidelining more than half the contenders, signals a new phase of the campaign, when the party can debate the issues that divide more mainstream Democrats, embodied by Biden, from an insurgent activist wing represented by Warren and Sanders.

To my eyes, it looks a lot like 2016, with Biden in the Hillary Clinton establishment role and Warren picking up the progressive mantle that Sanders carried all the way to the Democratic convention. Unlike the Clinton-Sanders brawl, that could be a healthy fight. And because Democrats want so badly to win, whichever candidate emerges victorious should be able to count on the other’s full-throated support, which wasn’t the case with Sanders versus Clinton.

“ It looks a lot like 2016, with Biden in the Hillary Clinton establishment role and Warren picking up the progressive mantle ”

Whoever emerges victorious, I don’t think we’ll see Biden people or Warren people sulking at the convention in Milwaukee in July the way the Sandersistas refused to give it up for Clinton in Philadelphia in 2016.

I put that argument to David Wasserman with the non-partisan Cook Political Report, and he quickly dismissed my rosy scenario. The shape of the election has not changed much in recent months, he said, except for the upward momentum for Warren. She and Biden appear to be the frontrunners, with Kamala Harris third and Sanders fourth, he says.

After that, there’s a big drop off for others in the field. Some analysts include Mayor Pete Buttigieg in the top five because he is well-funded and has staying power, but no one thinks he could win the nomination.

“I don’t see one figure able to unite the party,” Wasserman told the Daily Beast. “If Biden wins, he will have likely won the nomination in a very protracted primary and a possibly contested convention. Many young voters on the left don’t just oppose Biden, they actively dislike him.”

As for Warren, Wasserman says, “I don’t find much to persuade me she would do better than Hillary.” With Trump a master at exploiting differences and grievances, Wasserman says it’s easy to imagine ads that target Warren’s signature wealth tax on all assets over $50 million as the crazy idea of a Harvard professor who will be coming after farmers to assess the worth of their tractors.

As long as Sanders stays in the race, he draws from Warren, which helps Biden. Sanders has the money and the support to go the distance, but if he doesn’t do well enough in Iowa and loses New Hampshire, he could drop out before Super Tuesday and throw his support to Warren. That would make Sanders a kingmaker.

Warren goes into September riding a wave of positive media coverage. The press belatedly is picking up on the energy at her rallies. She drew 15,000 in Seattle on an August weekend, a crowd large enough to provoke Trump’s condemnation of the media for allegedly high-balling her crowds while failing to sufficiently give him credit for routinely attracting much bigger crowds.

“People are catching up on Warren as more than just an egghead wonk from Harvard, but a good retail politician,” says a Democrat who is consulting with more than one campaign.

As for Biden, the doubts about him are real, says another Democrat who doesn’t want to be quoted by name. “He is an energetic healthy man of his age—and I could add white to the list,” this Democrat says. “That’s why we have an intramural generational debate, a healthy one. To seal the deal, he’s got to show he can stand up to Trump, which shouldn’t be that hard.”

When Barack Obama chose Biden as his vice president in 2008, he was warned that the voluble senator was undisciplined and a loose cannon, and that he would be taking a chance with him. Obama weighed the pros and cons and went with Biden because of his connection to white working-class America, and his long experience in the Senate.

They became the best of friends, but that doesn’t mean that Biden has tamed his tongue. Anyone who has worked with Biden over his 30-plus years in public service knows how difficult it is to keep him from running his mouth, and to get him to stay on message and avoid gaffes.

The Washington Post last week found that Biden conflated stories about three war heroes, mixing facts and confusing timelines.

Biden told a South Carolina newspaper, The Post and Courier, that he didn’t understand what his critics are talking about because his “central point” about the heroism of today’s troops was “absolutely accurate.”

The story didn’t seem to have major impact. Biden was boosting others, not himself, and his errors of memory were arguably within the range of any retail storyteller, which is where Biden excels, and where he connects with his audience. Ronald Reagan spent World War II in Hollywood, yet he once told the Israeli foreign minister he had been at the liberation of Nazi death camps.

But this is a different time, and the gaffe police are on 24-hour surveillance of Biden. His campaign is limiting his exposure to unscripted events and husbanding his strength, which is too bad because Biden is a great retail politician. So far, voters are forgiving, this is the Biden they know and trust, and the African-American community is behind him even with Harris and Booker in the race. Black voters, and especially black women voters, are the core of the Democratic Party.

“If Biden loses African-American support, then he craters, and once he starts cratering, there’s no end,” says a Democrat giving voice to the one phenomenon that could end the Biden campaign. It wouldn’t take much for Harris to rise in South Carolina, and a race that seems settled on Labor Day with a familiar divide over incremental versus structural change could turn into a free-for-all.

One of the most important factors for the ultimate success of the eventual Democratic nominee is party unity. Reforms to the primary calendar, such as moving California and Texas up to Super Tuesday, were designed to produce an early winner. But the ideological differences and generational divisions on display among several well-funded candidates make party unity look like a pipe dream.