Sen. Bernie Sanders of Vermont officially endorsed former Vice President Joe Biden on Monday.

Sanders dropped out of the race on Wednesday after failing to pick up momentum in a series of vital primary elections, making Biden the presumptive Democratic nominee.

Since Sanders dropped out, Biden's campaign has made an effort to reach out to Sanders' base.

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Sen. Bernie Sanders of Vermont officially endorsed former Vice President Joe Biden in the 2020 presidential race on Monday.

"We've got to make Trump a one-term president, and we need you in the White House," Sanders said in a livestreamed video with Biden.

Sanders, a progressive firebrand who hit the ground running with a series of wins in early primary and caucus states, dropped out of the race on Wednesday after failing to pick up momentum, making Biden the presumptive Democratic nominee.

Biden also praised Sanders on Monday and said he would need Sanders "not just to win the campaign, but to govern."

"You've been the most powerful voice for a fairer and more just America," Biden told his former competitor, adding, "You don't get enough credit, Bernie, for being the voice that forces us to take a hard look in the mirror and ask ourselves, 'Have we done enough?'"

The two politicians also discussed how the country should respond to the coronavirus pandemic and economic crisis.

Sanders and his fellow progressives are attempting to extract policy concessions from Biden on a host of key issues, including healthcare, student debt, and climate change.

Since becoming the presumptive nominee, Biden has tried to make policy overtures to Sanders' supporters. Last week, he introduced a proposal to lower the eligibility for Medicare to age 60 from 65 and expand student-loan forgiveness to some low-income and middle-class people.

Rep. Alexandria Ocasio-Cortez, Sanders' highest-profile surrogate, has long pledged to support the 2020 Democratic nominee but has been sharply critical of Biden. Ocasio-Cortez made it clear in a recent interview with The New York Times that her endorsement of Biden would not come without "uncomfortable" policy shifts on Biden's part.

"There's this talk about unity as this kind of vague, kumbaya, kind of term," she said. "Unity and unifying isn't a feeling, it's a process."

She added: "The whole process of coming together should be uncomfortable for everyone involved — that's how you know it's working. And if Biden is only doing things he's comfortable with, then it's not enough."

Ocasio-Cortez said Biden should promise to provide a pathway to citizenship for the millions of undocumented immigrants living in the US and provide a more progressive plan to expand access to affordable healthcare.

The 2020 race was thrown into turmoil this year with the outbreak of the novel coronavirus, which causes a disease known as COVID-19. The World Health Organization designated the coronavirus outbreak a pandemic on March 11.

The virus originated in China's Hubei province late last year and has since spread to at least 185 countries and territories around the world. The US is the global epicenter of the outbreak, with more than 550,000 confirmed cases and more than 22,000 deaths as of Monday.

The US outbreak has sparked chaos in the electoral campaign season as states impose stay-at-home orders for residents and grapple with how to proceed with their primary contests.

Kayla Epstein contributed to this report.