Bennet announced that he was endorsing the former vice president hours after Sen. Bernie Sanders (I-Vt.) dropped out of the race.
Bennet suspended his own presidential bid in February after failing to break through the crowded Democratic primary field.
“Americans are asking two questions in this election: Who can beat Donald Trump, and who can get anything done? That candidate is former Vice President Joe Biden, and I am proud to endorse him for President of the United States,” Bennet said in a statement.
He hailed Biden as “a proven leader” whose “vast experience, temperament, and decency stand in stark contrast to those of Donald Trump, especially in these challenging times.”
Scaramucci, who soured on Trump after he was fired from his White House post after 11 days in the job, simply tweeted Biden’s campaign logo.
The Lincoln Project is an organization made up of Republicans that cites its mission as defeating “President Trump and Trumpism at the ballot box.” The most high-profile member is George Conway, husband to White House senior adviser Kellyanne Conway and frequent Trump critic.
“We are proud to endorse @JoeBiden for President,” the group tweeted. “As America contends with unprecedented loss, we need a leader who can steady the ship, heal our common wounds, and lead us into our next national chapter. Joe Biden has the humanity, empathy and steadiness we need in a leader.”
Michael Scherer contributed to this report.