Another two former presidential rivals have endorsed Joe Biden in the latest sign that the Democratic establishment is coalescing around the former vice president to stop the candidacy of Bernie Sanders.

Former Massachusetts Governor Deval Patrick and John Delaney, a former Maryland congressman – both at one time 2020 candidates – backed Biden, as did a slew of other Democratic officials in states soon to hold nominating contests.

Mr Sanders, who is desperate to regain some momentum after Mr Biden’s strong ‘Super Tuesday’ showing this week, launched a full-throated attack on his rival, assailing Mr Biden over his record on trade, abortion, gay rights and social security.

At a news conference in Phoenix, Arizona on Friday (local time), Mr Sanders criticised Mr Biden for having opposed the rights of gay people to serve in the US military and for voting against federal funding for abortions, stances the former vice president has since rejected.

“I was there on the right side of history, and my friend Joe Biden was not,” Mr Sanders said

The American people are sick and tired of endless wars in the Middle East which have cost hundreds of thousands of lives and trillions of dollars. We will not defeat Trump with a candidate like Joe Biden who supported and voted for the Iraq War alongside Republicans.

— Bernie Sanders (@BernieSanders) March 7, 2020

The pair contest six Democratic nominating contests on Tuesday, including the big prize of Michigan, with 125 of the 1,991 delegates needed to win the nomination at stake.

Ahead of Michigan’s primary, the state’s lieutenant governor, Garlin Gilchrist II, backed Biden, as did part of the state’s United Food and Commercial Workers Union, and former US Senator Carl Levin.

Mr Sanders lambasted Mr Biden for supporting trade deals he said had been “a disaster for Michigan” and accused Biden of trying in the past to cut Social Security, the government-run pension and disability program.

Mr Biden, who denies ever advocating cuts to Social Security, snapped back in a tweet on Friday: “Get real, Bernie. The only person who’s going to cut Social Security if he’s elected is Donald Trump. Maybe you should spend your time attacking him.”

Get real, Bernie. The only person who’s going to cut Social Security if he’s elected is Donald Trump. Maybe you should spend your time attacking him. https://t.co/7rwvXWmUWG

— Joe Biden (Text Join to 30330) (@JoeBiden) March 6, 2020

The exchange reflects mounting tension between the two White House hopefuls. The race became a tight two-way contest after US Senator Elizabeth Warren of Massachusetts and billionaire former New York Mayor Michael Bloomberg ended their White House bids after disappointing showings in the Super Tuesday primaries.

Mr Sanders, 78, said he would support Mr Biden, 77, if he becomes the Democratic nominee but insisted that only he, not Mr Biden, could “energise the American people” enough to beat Trump.

Bernie Sanders is looking to close the gap in pledged delegates. Photo: AP

Former South Bend, Indiana, Mayor Pete Buttigieg and US Senator Amy Klobuchar of Minnesota have already dropped out of the race and endorsed Biden.

The former vice president said at a private campaign event on Friday (local time) that his campaign had raised about $22 million over five days.

-AAP