Elizabeth Warren has endorsed Joe Biden, becoming the last of the former US vice president's major Democratic presidential rivals to formally back him.
The Massachusetts senator dropped out of the race in March, shortly after a disappointing third-place finish in her home state.
She refused to immediately endorse Biden or her fellow progressive Bernie Sanders.
But on Wednesday she backed Mr Biden as someone she said she believed could restore America's faith in goodness.
"In this moment of crisis, it’s more important than ever that the next president restores Americans’ faith in good, effective government—and I’ve seen Joe Biden help our nation rebuild," Ms Warren said.
"Today, I’m proud to endorse Joe Biden as President of the United States."
Mr Sanders, who suspended his campaign last week, endorsed Biden on Monday.
"We need you in the White House," Mr Sanders said to Mr Biden, adding that it was an imperative for the party to prevent Mr Trump from getting a second term.
A day later, former president Barack Obama announced his public backing of Biden.
Former President Barack Obama has endorsed Joe Biden, his two-term vice president. Source: ABACA
Mr Obama said he believed his longtime vice president had the ability to unify and "heal" a nation struggling through some of its darkest moments.
"Joe has the character and the experience to guide us through one of our darkest times and heal us through a long recovery," Obama said in a 12-minute video.
"I believe Joe has all the qualities we need in a president right now."
