WASHINGTON — Rep. Barbara Lee, D-Oakland, who has a reputation for voting against wars, now has a chance to broker peace in her own party.

Although Hillary Clinton has secured enough delegates to become the presumptive Democratic nominee for president, Sen. Bernie Sanders of Vermont has not endorsed his rival and has refused to concede defeat, holding out to make sure Clinton and the party shift their policy agenda to include many of his positions — from raising the minimum wage to enacting a carbon tax to providing tuition-free college and single-payer health care.

His last hope to push his party to the left is the party’s platform committee, whose members include Lee.

Lee, who endorsed neither Sanders nor Clinton in the primaries, said this week she hopes to use her neutrality to broker a deal that will keep Sanders and his supporters happy and preserve a party united in its effort to defeat Donald Trump, the Republicans’ presumptive nominee, in November.

“It’s a fairly heavy lift for all of us,” Lee said in an interview Wednesday. “So much is at stake, and we want to make sure Trump doesn’t win, so we have to have a platform that’s inclusive.”

Lee, a veteran of her party’s left, is one of just four members of the platform committee appointed by the chairwoman of the Democratic National Committee, Debbie Wasserman Schultz. Under the rules, Schultz could have appointed the entire 15-member committee.

Clinton’s struggles

Instead, Clinton appointed six members and Sanders selected five, an unusual proportional allotment that acknowledges the more than 12 million voters, many of them young, who flocked to Sanders over the long primary season that grew increasingly bitter up to its crescendo in California, where Clinton locked up the nomination with a decisive victory almost two weeks ago.

Clinton struggled to generate enthusiasm on the stump and very much wants Sanders voters on her side in November.

Sanders made it clear that his support will come at a price, pledging Tuesday in Washington to force delegates to the Democratic National Convention in Philadelphia next month to adopt “the most progressive platform ever passed by the Democratic Party.” And he reiterated that point in a speech Thursday. “This campaign has never been about any single candidate,” he told supporters. “It has been about transforming America.”

Healthy competition

Lee said the competition between Clinton and Sanders has been good for the party, as it has forced both to crystallize their views. She does not foresee policy fights “that would warrant divisiveness at least on our drafting committee, because Sen. Clinton has embraced many of the ideas that Sen. Sanders has put forth and Sen. Sanders has agreed with Sen. Clinton in a lot of ways.”

Joining Lee as party appointees are the chairman, Maryland Rep. Elijah Cummings; former Los Angeles Rep. Howard Berman; and Bonnie Schaefer, former CEO of Claire’s Stores.

Sanders’ members are Princeton Professor Emeritus Cornel West; Rep. Keith Ellison of Minnesota; climate activist Bill McKibben; James Zogby, head of the Arab American Institute; and Deborah Parker, an American Indian activist.

Clinton appointees are Ambassador Wendy Sherman; Neera Tanden, head of the Center for American Progress and a former Clinton aide; Rep. Alicia Reece of Ohio; Carol Browner, former chief of the Environmental Protection Agency; Rep. Luis Gutiérrez of Illinois; and union chief Paul Booth.

On Tuesday, after meeting with Clinton, Sanders called for a platform that would aid the “disappearing middle class, the 47 million people living in poverty, and take on the greed of powerful special interests who ... have so much power over the political and economic life of our country.”

Democrats have laid out an unusually open platform-writing process, soliciting public testimony in person, by writing or by video at https://demconvention.com/platform. The committee met publicly for two days in Washington last week, hearing from elected officials and citizens, and held another forum Friday in Phoenix. It will meet again late this week in St. Louis, with a final platform meeting in Orlando on July 8 and 9.

Lee said things are going smoothly so far. “Right now, people have different points of view, but it hasn’t been divisive or contentious,” she said. “I think everybody has a goal in mind of us defeating Trump.”

Carolyn Lochhead is The San Francisco Chronicle’s Washington correspondent. Email: clochhead@sfchronicle.com Twitter: carolynlochhead