In a surprise moment at the end of his Dallas rally on the eve of Super Tuesday, former Vice President Joe Biden called former Texas Rep. Beto O'Rourke up to the stage -- and vowed to put O'Rourke, who has said the government should forcibly seize assault rifles from Americans, in charge of gun-control efforts.

Also at the rally, Biden accepted the endorsements of onetime rivals Pete Buttigieg and Amy Klobuchar, as the Democratic party's moderate wing rallied behind Biden to challenge anti-establishment frontrunner and self-described "democratic socialist" Sen. Bernie Sanders, I-Vt.

"Ladies and gentlemen, tomorrow, March 3, 2020, I will be casting my ballot for Joe Biden," O'Rourke announced to applause at the rally's conclusion.

O'Rourke called President Trump an "existential threat" to "free and fair elections," and urged rallygoers to view Biden as the "antithesis of Donald Trump," someone who is "decent, kind, caring, and empathetic."

Biden, taking the microphone after O'Rourke spoke, then announced, "I want to make something clear -- I'm gonna guarantee you, this is not the last you're seeing of this guy -- you're gonna take care of the gun problem with me, you're gonna be the one who leads this effort. I'm counting on you, I'm counting on you, we need you badly."

O'Rourke has previously said he would like to seize all Americans' AR-15 assault rifles, promising: "Hell, yes, we're going to take your AR-15, your AK-47," at a Democratic primary debate last year, shortly before his campaign collapsed.

"Yikes," remarked Trump campaign spokesman Andrew Clark.

"Joe Biden promises Beto O'Rourke, who pushed for gun confiscation for legal gun owners, will 'be the one who leads' his gun control effort," observed GOP Rapid Response Director Steve Guest. "2020 Democrats are unambiguous about their anti-Second Amendment agenda."

Wrote NBC News' Benjy Sarlin: "Biden hugging Beto going 'You’re going to take care of the gun problem for me!' is a thing you will see in a Republican ad someday."

The moment was yet another headache for Biden as he struggles to appeal to gun owners. Last week, he bizarrely declared that "150 million" Americans -- approximately half the country -- had died due to gun violence since 2007.

Separately, Biden spent most of his time attacking President Trump, rather than his remaining rivals Elizabeth Warren, Sanders and Mike Bloomberg.

After Sen. Klobuchar, D-Minn., spoke earlier in the rally, Biden immediately sought to draw a contrast between her and Trump, saying the White House entirely lacks "empathy and decency."

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"Folks, I knew -- I believed that he wasn't going to be a very good president," Biden said. "But I have to admit to you that Donald Trump, well, I didn't have any idea how much it was always going to be about Donald Trump. It's having a corrosive impact. It's having a corrosive impact on our children."

Biden continued: "The days of Donald Trump's divisiveness will soon be over. Folks, there's two ways people get inspired. They get inspired by great leaders like Lincoln and Roosevelt and Kennedy and Obama, but they also get inspired by very bad leaders. No, I really mean it. This president has sort of ripped the band-aid off, exposed just how venal he's become, and how he has literally strangled the life out of the Republican Party."

"You're gonna take care of the gun problem with me." — Joe Biden to Beto O'Rourke

At the same time, in Minnesota, Sanders and top surrogate Democratic Rep. Ilhan Omar were rallying a boisterous crowd at a competing rally. Within hours, 14 states and one U.S. territory will head to the polls in contests that will award a whopping one-third of total delegates to the Democratic National Convention.

"It looks like St. Paul is ready for a political revolution," Sanders said to cheers.

At the rally in delegate-rich Texas, however, Biden and his new supporters argued that most voters want "evolution" and not a drastic change. Buoyed by his emphatic victory in Saturday's South Carolina primary, Biden has argued he's the only candidate who can realistically win over minority support and defeat Trump.

Klobuchar, who formally suspended her campaign earlier Monday, declared that Biden was the only sensible choice for voters who feel "tired of the noise and the nonsense" of today's politics.

"How great is it to be here in Dallas, Texas?" Klobuchar asked. "And how great it is to be here with my family. ... And with Vice President Biden! And how wonderful it is to be standing next to him on a stage when it isn't the debate stage?" (Biden later joked that Klobuchar had "won all the debates.")

It's time, Klobuchar said, for Americans to "join hands instead of pointing fingers."

"We need to unite our party and our country, and to do it not just with our words but with our actions," Klobuchar continued. "It is up to us, all of us, to put our country back together, to heal this country, and then to build something even greater. I believe we can do this together. And that is why, today, I am ending my campaign and endorsing Joe Biden for president."

Klobuchar and Biden then hugged as cheers of "Let's go, Joe!" broke out. Later, Klobuchar noted that Minnesota was also voting on Tuesday, and urged voters there to support Biden as well, amid concerns that her sudden withdrawal might actually help Sanders win the state.

Biden went on to thank Buttigieg for his endorsement earlier in the day as well. Speaking to supporters, Biden said Pete reminded him of his late son Beau during the debates -- a compliment Biden then falsely said he had not made before about any other politician.

"I’m looking for a leader, I’m looking for a president, who will draw out what’s best in each of us,” Buttigieg, who ended his campaign Sunday, told reporters prior to the rally. “We have found that leader in vice president, soon-to-be president, Joe Biden.”

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In Minnesota, Sanders praised both Buttigieg and Klobuchar even as the two were in Texas to boost Biden.

"Tonight, I want to open the door to Amy's supporters, to Pete's supporters," he said. "I know there are political differences, but I also know that virtually all of Amy's supporters and Pete's supporters understand that we've got to move toward a government that believes in justice, not greed."

"I’ve known Amy for a very long time, and she is one of the hardest workers I know," Sanders said of Klobuchar before going on to call Buttigieg's campaign "historic" and "brave," noting that Buttigieg would have been the first openly gay president.

Biden currently trails Sanders by just eight pledged delegates (56 to 48) in the race to get the 1,991 necessary to secure the Democratic party's nomination at this summer's convention in Milwaukee. Party leaders had feared that the splintering moderate vote would allow Sanders to rack up victories in a series of high-stakes Super Tuesday contests and possibly gain an insurmountable advantage in the delegate count.

However, former New York City Mayor Michael Bloomberg has resisted calls to clear the path for Biden, predicting Monday that Democrats would have a contested convention.

"The most likely scenario for the Democratic Party is no one has a majority -- it goes to a convention where there's horse-trading; there's compromise," Bloomberg said at a Fox News town hall in Manassas, Va., one of the states where voters will head to the polls Tuesday. "It doesn't even have to be the leading candidate; it could be the one with a smaller number of delegates."

Despite his endorsements and win by a massive margin in South Carolina's primary on Saturday, Biden remains vulnerable on a variety of fronts, including his repeated gaffes. On Monday, he slipped up twice during an earlier rally -- at one point, badly garbling the Declaration of Independence before giving up, and at another, saying "Super Thursday" was coming up.

And, although the Biden campaign reported back-to-back days of $5 million fundraising hauls following his South Carolina win, the former vice president has struggled to raise money more broadly.

In Silicon Valley, which dominated by the tech sector, many wealthy donors prioritize executing a data-driven plan -- and Biden’s rocky campaign pushed many toward Buttigieg or Bloomberg in recent months, financiers say.

Biden's team is waging a quiet campaign to win them over, yet many are taking a wait-and-see approach.

At Monday's gaffe-riddled rally, Biden notably didn't take any shots at Bloomberg and instead aimed his barbs at Sanders -- in an apparent sign that Bloomberg isn't viewed as a significant threat.

For his part, as midnight approached, Trump weighed in on Twitter to make clear he felt Biden's misstatements were very much a liability.

"WOW!" Trump wrote. "Sleepy Joe doesn’t know where he is, or what he’s doing. Honestly, I don’t think he even knows what office he's running for!"

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Fox News' Andrew O'Reilly and The Associated Press contributed to this report.