Many said that Sanders will have to take to the trail and campaign in earnest if he hopes to sway some of his supporters to Clinton's side.
Bernie Sanders has formally backed Hillary Clinton for the Democratic party's candidate for president during a rally in Portsmith, New Hampshire on Tuesday. She later went on to applaud him for all the he was able to accomplish with his campaign and thank his many supporters.
However, Donald Trump, the presumptive Republican presidential challenger, anticipated the official announcement and chimed in early on Twitter, saying Sanders had "sold out" to "Crooked Hillary Clinton".
Stein - who has always been a vocal critic of Clinton - disagreed, tweeting, "many Bernie hearts are breaking right now" and including an emoji of a breaking heart.
Mrs Clinton and Mr Sanders had been negotiating since she all but guaranteed the nomination in June.
Trump shot back with several more tweets slamming Sanders for his endorsement of Hillary Clinton and inviting the Vermont senator's supporters to back the Trump campaign. Bernie Sanders, I-Vt., who battled Clinton during a long, hard-fought Democratic primary.
"Hillary Clinton will make an outstanding president, and I am proud to stand with her here today", he said.
"I'm at a loss for how you make that jump from Sanders to Trump", Carroll added.
The endorsement comes more than a month after Clinton clinched enough delegates to secure the Democratic nomination.
Democrats familiar with the plans said Sanders will publicly endorse Clinton's White House bid after weeks of negotiations between the Clinton and Sanders campaigns.
Demonstrators gathered outside a rally Tuesday in Portsmouth where U.S. Sen.
Host Eddie Arruza talks about all of the latest campaign news with Tom Bowen, a political consultant at New Chicago Consulting LLC; Clem Balanoff, who served as the IL state director for the Sanders campaign; Anthony Anderson, an IL delegate for Trump; and Ed Lasky, co-founder of the American Thinker.