Those predictions have been proved wrong. Tuesday’s Democratic primary debate in Ohio will feature former vice president Joe Biden in his usual position at center stage, mirroring his place in the “center lane” of the nomination contest. He will be flanked on either side by Sanders and Warren, who have emerged as his top two rivals for the nomination (with Warren now arguably the front-runner). And they will be competing in a race that, as my Nation colleague D.D. Guttenplan argues, is “dramatically widening the entire left lane of American politics.”

Sanders and Warren are upending the traditional downsized politics of excluded alternatives. Their support for policies including Medicare-for-all, a Green New Deal, a wealth tax, student-debt cancellation and more has expanded widely held notions of what is possible. These ideas have not just been mentioned in the mainstream debate; thanks to Sanders’s and Warren’s courage and clarity of vision, they have dominated the discourse. This has often left the rest of the field flailing to defend tired centrist policies or, in some cases, scrambling to move left in an attempt to keep up.

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The progressive senators’ success has also stemmed in part from a decision to set aside their differences and join forces against the pro-corporate Democratic establishment. While Sanders is an avowed socialist and Warren an unabashed progressive capitalist, they share a commitment to reining in corporate power and a mutual respect. That tacit alliance was clear at the last debate when Sanders and Warren teamed up to bat down misleading attacks on Medicare-for-all and redirect the focus to the perfidies of the insurance industry. They have been similarly in sync on key economic issues, such as trade and workers’ rights.

What’s even more remarkable is that the two senators have dominated the debate while relying exclusively on grass-roots donors to fund their campaigns. Both candidates have sworn off PAC donations and high-dollar fundraisers. Yet they still raised significantly more in the third quarter of the year than their closest opponents, with Sanders and Warren bringing in $25.3 million and $24.6 million, respectively. This impressive showing is subverting the conventional wisdom about what it takes to compete in a national race and proving that Democrats can untether their campaigns from lobbyists and corporate power, a prerequisite (along with winning the Senate) to derigging our money-drenched political system and realizing many progressive goals.

It would be naive, however, to expect this momentum to continue on its own. In the near term, the impeachment inquiry in the House of Representatives could take energy and attention from the ideas and movements that have shaped the primary to date. Some are waging a campaign to drive a wedge between the Sanders and Warren camps that, if successful, could severely deflate the energy among progressives. And as Sanders scales back his activity in the wake of his heart attack, Warren is confronting a new wave of personal attacks on what seems like a daily basis.

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As the campaign enters its next phase, it’s important that progressives remain united and remember who their real opponent is. That will demand that Sanders and Warren keep their truce and continue to compete through the boldness of their ideas rather than clash with each other. Meanwhile, the pair’s supporters might take a cue from Sanders, who used his recent hospitalization as an opportunity to renew the focus on the issues at the heart of his campaign. “Be proud of the efforts that we’re making,” he said last week in a message to supporters. “Understand the enormous opposition that we’re facing from the drug companies and the insurance companies, but we are going to win this struggle. History is on our side.”