With just hours to go before Super Tuesday, the moderate wing of the Democratic party attempted to unite as a group of former candidates flocked to Texas and endorsed Joe Biden over his main rival, the progressive frontrunner Bernie Sanders.

At a raucous campaign rally in Dallas, Amy Klobuchar and Beto O’Rourke offered their backing to the former vice-president. Hours earlier, outside a fast food restaurant in the same city, Pete Buttigieg also offered his endorsement.

Super Tuesday, which sees 14 states and two other constituencies weigh in on the Democratic primary, marks a crucial juncture in the race for the presidential nomination. There are more than 1,300 delegates up for grabs, over a third of the entire tally in the election, with the race now widely viewed as a battle between Biden and Sanders, with the Massachusetts senator Elizabeth Warren and the billionaire Mike Bloomberg lagging behind.

Sanders holds a strong lead over Biden and Warren in the delegate-heavy state of California, but the race in Texas, which offers the second-largest delegate haul, is much closer, along with other battles in North Carolina and Virginia, where polls suggest Biden has a slight lead and Bloomberg remains competitive.

Klobuchar and Buttigieg’s last-minute exits from the race present further unknowns going into the vote, and their endorsements of Biden, a fellow centrist Democrat, provide a major boost to his campaign.

Klobuchar, having ended her presidential run hours earlier, appeared on stage with Biden in front of a packed crowd. “I couldn’t think of a better way to end my campaign than joining his,” she said, her voice cracking, as she paid tribute to her campaign staff. She urged the Democratic party to unite around him.

“We need to unite our party and our country and to do it not just with our words, but with our actions,” she said.

Facebook Twitter Pinterest Beto O’Rourke endorses Biden. Photograph: Richard W Rodriguez/AP

Buttigieg had flown to Texas from South Bend on Monday evening and told an impromptu event in Dallas: “When I ran for president, we made it clear that the whole idea was about rallying the country together to beat Donald Trump.” Flanked by Biden, who embraced him as he finished talking, Buttigieg added: “And it is in the name of that very same goal that I am delighted to endorse and support Joe Biden for president.”

As Biden received the high profile endorsements, Sanders addressed a huge rally in St Paul, Minnesota, and paid tribute to his former competitors, inviting their supporters to join his campaign.

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“For all of Amy and Pete’s millions of supporters: the door is open, come on in,” he said, paying tribute to Buttigieg’s “historic… and brave campaign” and noting the former mayor was the first openly gay party candidate for the presidency. He described Klobuchar as “one of the hardest workers I know”.

Warren also held a huge rally on Monday night in Los Angeles, a reflection of her strong support in California, while Bloomberg held a town hall event in Virginia that was interrupted by protesters. Tuesday marks the first time Bloomberg will appear on the ballot after he invested $500m of his personal fortune in his campaign.

But in a battle for optics ahead of the critical vote, Biden’s campaign appear to win. Flanked by Klobuchar, he took to the stage as thousands of supporters chanted his name.

“Just a few days ago the press and the pundits declared this campaign dead, but South Carolina had something to say about it,” he said, referencing his landslide victory on Saturday that breathed new life into a flailing campaign. “And tomorrow Texas and Minnesota and the rest of the Super Tuesday states – they’re going to have a lot to say about it.”

As he wrapped a stump speech, he introduced O’Rourke, the former Texas congressman whose short run for president ended last November after poor polling and a disastrous launch announcement on the cover of Vanity Fair.

O’Rourke remains popular among Democrats in his home state, however, who cheered as he entered the stage.

“I will be casting my ballot for Joe Biden,” he told the crowd. “Let me tell you why: we need somebody who can beat Donald Trump.”

Biden later suggested O’Rourke could have a future role in his campaign or administration working on gun control, an issue he pushed hard during his campaign.

Determined not to be upstaged, Donald Trump held a campaign rally in Charlotte, North Carolina, a Super Tuesday state where polls show a tight contest between Sanders, Biden and Bloomberg.

“We like to troll,” the president told a packed sports arena. “We like to go the night before one of their primaries and we do a little trolling. It’s called, ‘We do a little trolling.’ Bernie Sanders was very upset: ‘Why would he be there? Why?’ Because I want to win. That’s a good reason.”