WASHINGTON — Bay Area Reps. Barbara Lee and Ro Khanna endorsed Joe Biden for president Monday along with other progressive lawmakers, a sign that the former vice president is further consolidating Democratic support for his candidacy.

The presumptive nominee has already been endorsed by his last remaining rival for the Democratic nomination, Vermont independent Sen. Bernie Sanders, but questions remain whether the moderate candidate can energize the left flank of the base enough to defeat President Trump in November.

Lee, D-Oakland, and Khanna, D-Fremont, were joined by colleagues from across the country in pledging to support Biden and help excite progressive voters.

“As strong progressive Democrats who supported different candidates or did not support any candidate during the primary for the Democratic nomination, we recognize that the future of our democracy is at stake,” the group wrote. “We are not simply endorsing Vice President Joe Biden today. We are pledging to initiate a wave of progressive organizing on issues we care about so passionately. Together we must turn the country towards a fundamentally new direction of equality, prosperity and justice.”

Both Lee and Khanna had served in high-level positions on other Democratic candidates’ campaigns.

Lee served as the California co-chair for the campaign of California Sen. Kamala Harris, and Khanna was a national co-chair for Sanders’ campaign. Harris suspended her campaign in December, and Sanders conceded the race was unwinnable for him this month.

Democrats fear that progressive apathy or distaste about Biden could cost him the presidential race. Sanders appeared with Biden in a live stream to make his endorsement, after some criticized him for not doing enough to rally his supporters to Hillary Clinton’s side in the 2016 election.

Biden has taken more moderate positions during the primary, including arguing against progressives’ rallying cry of Medicare For All.

Biden also won the endorsement Monday of the Congressional Black Caucus PAC, the political arm of a group of African American lawmakers in Washington. His support among black voters helped him defeat his fellow Democrats in the primaries.

In their statement on supporting Biden, the progressives made clear that any alternative would empower Trump to a second term.

“The threat of a second presidency of Donald Trump requires all of us to support Vice President Joe Biden,” they wrote. “Now more than ever we must fight to restore decency, inclusion and competency to the White House.”

Tal Kopan is The San Francisco Chronicle’s Washington correspondent. Email: tal.kopan@sfchronicle.com Twitter: @talkopan