Tom Perez, the former Obama administration secretary of labor, was elected Saturday to chair a Democratic National Committee that must rapidly renew the party after a devastating 2016 election cycle that saw Republicans take control of the executive and legislative branches of the federal government and most statehouses.

“A united Democratic Party is not only our best hope,” declared Perez, “it is Donald Trump’s worst nightmare.” That is undoubtedly true.

But Perez clearly recognizes that he will have to work hard to first unite the party and then transform it into a dramatically more grassroots-oriented and ideologically progressive political force within a broader resistance to Trump.

Perez won the chairmanship by a narrow margin on the second ballot. The Atlanta gathering where the vote was held featured much talk about unity, but also recalled genuine divisions over how the Democratic Party should proceed. A strong backer of Hillary Clinton’s campaign for the 2016 Democratic nomination, Perez beat Minnesota Congressman Keith Ellison, a key backer of Bernie Sanders in that race, by just 35 votes out of 435 cast.

The new chair made an immediate unity move by announcing that he wanted Ellison to serve as deputy chair of the DNC. DNC members approved the designation by acclamation.

Ellison addressed the crowd, saying, “We don’t have the luxury, folks, to walk out of this room divided…. we’ve got earnest and sincere work to do.” Perez gave every indication that he heard the Minnesotan.

The new chair went out of his way to embrace that message, referencing his rival throughout an acceptable speech that declared a united Democratic Party would “lead the resistance to Donald Trump.”

The Perez-Ellison contest had echoes of the Clinton-Sanders primary race. A number of key Clinton backers lined up behind Ellison, and Perez had the backing of a number of prominent progressives, but Perez was surely seen as the candidate of the party establishment. Ellison, on the other hand, was seen as the candidate of insurgents who believe the DNC must become less reliant on big money and DC strategists and more reliant on small donors and grassroots organizing.

In his final speech to the 447 DNC members, Perez spoke about the need for “culture change” at the DNC, which is indeed necessary, but was short on specifics. Ellison was far more focused. He spoke about relying on small donations rather than big money to build a mass party, and about the need to link the “demonstration energy” of the resistance to Trump with the “electoral energy” of the party in 2018 and 2020.

The problem for Ellison, of course, was that he was delivering an anti-establishment message to what is, by definition, the party establishment. (Before Saturday’s vote for party chair, the gathered DNC members blocked a move to bar contributions from corporate lobbyists.) As such, the congressman’s bid for the party chairmanship was always an uphill one.

Yet Ellison came within a whisker of winning. And he filled the room with activists who, even as the selection of Perez was announced, chanted: “Party for the people, not big money!” Many in the room, like Working Families Party National Director Dan Cantor, argued that, “As chair, Tom Perez would be well-advised to take Keith Ellison’s counsel, and avoid that of the donor class that led us into the current crisis.”

Perez eased tensions by naming Ellison to the deputy-chair position, and by inviting the congressman to join him in his first meeting with reporters as party chair.

The former labor secretary was on the left of the Obama administration, and he signaled in his address to DNC members he recognizes the need for a more activist organizing focus on the part of a DNC that has been frequently criticized for being to0 bureaucratic and too DC-focused. Perez supporters wore T-shirts that read: “Stop Fretting, Start Fighting.”

Fighting is the key. But it must be more than a fight against Trump. It must be a fight for the aggressively progressive vision that provides a clear alternative to Trumpism. As Sanders said immediately after the vote, “It’s imperative Tom understands that the same-old, same-old isn’t working and that we must bring in working and young people in a new way. The Democratic Party must make clear it will stand up to the 1% and lead in the fight for social, racial, economic and environmental justice.”

In the Trump era, the DNC must be more than a traditional opposition party. It must, as Ellison proposed, align itself with the burgeoning grassroots resistance to the new administration and its allies in Congress. Perez has to come out fighting. But to fight right, Perez must ally with Sanders, Ellison and grassroots activists who are more concerned with principles than partisanship. This is not just about party unity. This is about a unity of progressive vision that the Democratic Party has struggled to achieve in the past, and that Tom Perez now has a duty to establish.