Bernie Sanders launched a full-throttle attack against Joe Biden on Friday, assailing his Democratic presidential rival over his record on trade, abortion, gay rights and Social Security, as the pair face a slew of crucial nominating contests next week.

Sanders, until recently the front runner in the party's race to face Republican President Donald Trump in November, now trails in terms of delegates numbers and is desperate to regain momentum after Biden's strong Super Tuesday showing this week.

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At a hastily arranged news conference in Phoenix, Arizona, Sanders dug deep into Biden's 40-year record. The U.S. Senator from Vermont criticized Biden for having opposed the rights of gay people to serve in the U.S. 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," Sanders said.

Sanders lambasted 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.

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."

The exchange reflects mounting tension between the two White House hopefuls in what has become a tight two-way race since U.S. 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.

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

Warren's exit means that what had been hailed as the most diverse field of candidates in U.S. history narrowed to a race for the nomination between two white, septuagenarian men. Tulsi Gabbard, a congresswoman from Hawaii with virtually no chance of winning, is the only other remaining Democratic candidate.

On Friday, the Democratic National Committee, which oversees the party's presidential debates, released new qualifying thresholds for the next debate in Arizona on March 15. Candidates will need at least 20 per cent of delegates awarded so far, essentially excluding Gabbard, who has won less than 1 percent.

Biden's Super Tuesday turnaround benefited as the Democratic Party establishment mobilized to try and stop Sanders, a democratic socialist, with former South Bend, Indiana, Mayor Pete Buttigieg and U.S. Senator Amy Klobuchar of Minnesota dropping out of the race and endorsing Biden.

Onetime Democratic presidential rivals Deval Patrick, the former Massachusetts governor, and John Delaney, a former congressman from Maryland, announced their support for Biden.

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 U.S. Senator Carl Levin of Michigan.

Biden also received endorsements in other states with upcoming primary contests, including from Seattle Mayor Jenny Durkan, in Washington state, and Ruben Gallego, an Arizona congressman. 

Chicago Mayor Lori Lightfoot and Senator Dick Durbin of Illinois are expected to soon announce endorsements of Biden, according to a source familiar with the matter. Illinois and Arizona vote on March 17, along with Florida and Ohio. 

A big win for Biden in delegate-rich Michigan on Tuesday would deal another major blow to Sanders' hopes of becoming the Democratic nominee. Four other states will hold primary elections on Tuesday: Idaho, Mississippi, Missouri and Washington state. North Dakota will hold caucuses.