Joe Biden’s campaign for the White House received another significant lift on Sunday when Democratic senator Kamala Harris, until recently one of the former vice-president’s main rivals for the party’s presidential nomination, announced her endorsement.

In a short video statement released on Twitter, Harris promised she would “do everything in my power” to ensure Biden beats off a challenge from Vermont senator Bernie Sanders for the nomination and goes on to be elected president in November.

“I am with great enthusiasm going to endorse Joe Biden for president of the United States,” the California senator said. “I believe in Joe. I really believe in him and I have known him for a long time.

Kamala Harris (@KamalaHarris) .@JoeBiden has served our country with dignity and we need him now more than ever. I will do everything in my power to help elect him the next President of the United States. pic.twitter.com/DbB2fGWpaa

“One of the things we need most now is a leader who really does care about the people and who can therefore unify the people.”

Biden, she said, was a public servant who had “always worked for the best of who we are as a nation, and we need that right now.”

Harris’s support means that Biden, whose faltering campaign for the nomination was restored by a remarkable Super Tuesday turnaround, goes into Tuesday’s primary contests in a number of crucial midwest and western states, including Michigan and Missouri, with the backing of almost all of the key figures previously running against him.

Pete Buttigieg, the former mayor of South Bend, Indiana, and Minnesota senator Amy Klobuchar dropped out last weekend and threw Biden their endorsements just before 14 states went to the polls on Super Tuesday.

Billionaire businessman Michael Bloomberg, the former mayor of New York, suffered a disappointing Super Tuesday and withdrew on Wednesday after spending $500m on his campaign. He immediately offered his backing and resources to the Biden operation.

Meanwhile Elizabeth Warren, the liberal Massachusetts senator who dropped out on Thursday, has yet to announce an endorsement, with Sanders hoping she will swing her supporters to him.

The Vermont senator, 78, also picked up one notable endorsement on Sunday, that of Jesse Jackson, the veteran civil rights leader who won the Michigan caucuses in his unsuccessful 1988 run at the Democratic presidential nomination.

“A people far behind cannot catch up choosing the most moderate path,” Jackson said in a statement. “The most progressive social and economic path gives us the best chance to catch up and Bernie Sanders represents the most progressive path.”

Harris, once seen as a likely frontrunner, ended her own campaign in December after struggling for months to overcome low opinion poll numbers. In a tweeted announcement to supporters she said running for president had been the “honor of my life” and promised to “keep up the fight”.

Harris has previously been severely critical of Biden, notably during a contentious debate in July last year when she attacked his record on race relations. Biden, she claimed, worked with segregationist leaders in southern states during the civil rights era and opposed “busing” of school children to achieve racial integration, claims Biden strenuously denied.

Biden, after his extraordinary Super Tuesday turnaround, holds a narrow advantage over Sanders in the chase for Democratic delegates who will decide the nomination at the party’s convention in Milwaukee in July.

The former Delaware senator and Barack Obama’s vice-president for eight years, has so far secured 664 of the 1,991 state delegates he needs, with Sanders – who raced out to an early lead after victories in Nevada and New Hampshire, has 573.

On Tuesday a further 352 delegates are at stake in primary votes in Idaho, Michigan, Mississippi, Missouri, North Dakota and Washington.

• This article was amended on 10 March 2020. The claim made by Kamala Harris which Joe Biden strenuously denied was not that he had “supported ‘busing’ of school children”, as an earlier version had said, but that he had opposed it. This has been corrected.