Sen. Bernie Sanders (I-VT) endorsed former Vice President Joe Biden’s presidential bid on Monday afternoon.

“We’ve got to make Trump a one term president and we need you in the White House,” Sanders said in a joint livestream event with Biden, the presumptive Democratic nominee. “I will do all that I can to see that that happens, Joe. It’s imperative that all of us work together.”

The Vermont senator’s endorsement comes as Biden has in recent days promised to make a concerted effort to woo Sanders’ progressive base. Sanders dropped out of the 2020 race on April 8 after lagging in the delegates needed to secure the Democratic nomination.

Biden, 77, and Sanders, 78, are longtime Senate colleagues and regularly refer to each other as “good friends.” Throughout the presidential primary, the two jousted on the debate stage but largely avoided hitting each other as hard as other rivals.

In their joint appearance on Monday, both Democrats acknowledged that they don’t always agree on every policy issue, with Biden preferring a more traditional moderate approach to Sanders’ brand of Democratic socialism, but that they are united in their effort to defeat President Donald Trump in November.

“Your endorsement means a great deal,” Biden, who referred to Sanders as the “most powerful voice” for young progressives, said. “I’m going to need you, not just to win the campaign but to govern.”

Top Biden officials, senior Sanders aides and outside allies were quick to surface the endorsement as a positive step in helping to push out Trump after his first term in office.

“It takes both wings to fly. Thanks @BernieSanders and @JoeBiden for your commitment to bring all shades of blue to the table. This is how we win while reforming the party,” Jane Fleming Kleeb, a board member for the progressive organization Our Revolution that was backing Sanders’ bid, wrote shortly after the announcement became public.

But not everyone was equally enthusiastic. Briahna Joy Gray, Sanders’ former National Press Secretary, was among the first to express her displeasure over the endorsement by her former boss.

“With the utmost respect for Bernie Sanders, who is an incredible human being & a genuine inspiration, I don't endorse Joe Biden,” she wrote in a series of tweets. “Joe Biden should also consider reaching out to his supporters and the supporters of other candidates, who have been more toxic, racist, and misogynistic than anything I have ever seen from a person purporting to back Bernie,” she wrote in another tweet. “This is not how unity is forged.”

One of the most prominent voices in Sandersworld used fewer words, but appeared to be just as upset. “Shoot Me Now,” RoseAnn DeMoro, a close friend of the senator and the former leader of the National Nurses United union, wrote minutes after the endorsement. DeMoro was considered to be one of the leading voices who had urged Sanders to stay in the race.

Other figures, including Young Turks host Cenk Uygur, who endorsed Sanders, hedged.

“There has to be something in between ‘endorsing’ a candidate & saying you're going to vote for them. @BernieSanders ran for president in Democratic Party, so I guess he had to endorse Biden, which he just did. For my purposes, I might vote for him without saying I endorse him,” he wrote.