(Reuters) - Two former presidential rivals endorsed Joe Biden on Friday in the latest sign that the Democratic establishment is coalescing around the former vice president to stop the candidacy of Bernie Sanders, who ratcheted up attacks on his rival ahead of crucial contests next week.

FILE PHOTO: Democratic U.S. presidential candidate and former Vice President Joe Biden speaks during a campaign stop in Los Angeles, California, U.S., March 4, 2020. REUTERS/Mike Blake

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

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

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.

Sanders, a U.S. senator from Vermont who was until recently the front-runner in the party’s race to face Republican President Donald Trump in November, now trails in delegates. A big win for Biden in Michigan would deliver another major blow to Sanders’ hopes of becoming the nominee.

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.

Four other states will hold primary elections on Tuesday: Idaho, Mississippi, Missouri and Washington state. North Dakota will hold caucuses.

At a news conference in Phoenix, Arizona, on Friday, Sanders dug deep into Biden’s 40-year record. He 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.

Slideshow ( 7 images )

Sanders also 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. The race became a tight two-way contest after 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.

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 meant 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 Mar. 15. Candidates will need at least 20% 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 began mobilizing this week to try and stop Sanders, a democratic socialist. Former South Bend, Indiana, Mayor Pete Buttigieg and U.S. Senator Amy Klobuchar of Minnesota dropped out of the race and endorsed Biden.

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

Biden also received endorsements on Friday 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.