(CNN) Sen. Amy Klobuchar ended her presidential bid on Monday and endorsed former Vice President Joe Biden for president.

"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," Klobuchar said at a Biden campaign event in Dallas, Texas.

"I believe we can do this together, and that is why today I am ending my campaign and endorsing Joe Biden for president," the Minnesota senator said.

Klobuchar said, "(Biden) can bring our country together and build that coalition of our fired-up Democratic base, and it is fired up, as well as Independents and moderate Republicans, because we do not in our party want to just eek by a victory. We want to win big. And Joe Biden can do that."

The senator's endorsement comes on the eve of the critical Super Tuesday contests, and shortly after former South Bend, Indiana, Mayor Pete Buttigieg endorsed Biden. Former Texas Rep. Beto O'Rourke also endorsed Biden the same night.

Klobuchar's path to the nomination all but closed after she posted sixth-place finishes in Nevada and South Carolina, a sign that the Minnesota senator's surprising showing in New Hampshire would not be nearly enough to propel her toward the nomination.

A Democratic official told CNN that the Klobuchar campaign was worried that the senator would lose her home state of Minnesota on Tuesday. Vermont Sen. Bernie Sanders, the race's front-runner, is holding a rally in the state on Monday night.

A senior Klobuchar adviser told CNN that the senator could have moved forward to try and win Minnesota or move to unify the party behind Biden, and ultimately chose the latter.

Conversations between Klobuchar and her campaign manager, Justin Buoen, about exiting the race and endorsing Biden started yesterday morning, the senior adviser said. Klobuchar was thinking about this move back before South Carolina, but real conversations started yesterday morning.

The high point of Klobuchar's campaign came in mid-February, when a strong debate days before the New Hampshire primary led to a third-place finish in the state. But the showing even caught Klobuchar's campaign off guard, and a lack of organization on the ground in Nevada and South Carolina, along with the senator's inability to win over Latino and black voters, ultimately stalled her candidacy.

Klobuchar launched her campaign pledging to focus on the "nation's heartland," hoping to use her status as a Midwesterner to convince Democrats that the best way to defeat President Donald Trump is to nominate someone who could bring back disaffected voters in places like Michigan and Wisconsin. On the campaign trail, she frequently joked about building a "Blue Wall" around the Midwest and making "Donald Trump pay for it." Those two states, which backed Barack Obama and then flipped to Trump, will be critical in the general election.

"I have the receipts," Klobuchar repeatedly said in the close of her campaign, pointing to the fact that she won some of Minnesota's more rural, conservative counties.

That strategy led Klobuchar to devote much of her campaign to Iowa, staking any future success on her ability to win over voters there who, in many ways, share cultural similarities with voters in Minnesota.

The senator, though, turned in a fifth-place finish in Iowa.

That left Klobuchar weakened in New Hampshire until, days before voters went to the polls there, the senator delivered an electric debate performance that raised her campaign millions, filled her following campaign events with people and gave her enocugh momentum to notch a third-place finish in the state.

"We are taking this campaign to Nevada. We are going to South Carolina. And we are taking this message of unity to the country," she said on the night of that primary, the apex of her campaign.

But Klobuchar was never able to fully nationalize her campaign because she constantly trailed her competitors in fundraising. Klobuchar entered 2020 with less than $5 million in the bank. While she received an injection of $12 million following her performance in New Hampshire, it was too late for her to turn that into useful television ads of operatives on the ground.

Because of those money struggles, Klobuchar's campaign had little organization on the ground in Nevada, forcing her to scramble staff from across the country to the Silver State in an effort to turn her New Hampshire momentum into a tangible operation. It was for naught -- Klobuchar took a distant sixth place there.

Photos: Former presidential candidate Amy Klobuchar Photos: Former presidential candidate Amy Klobuchar US Sen. Amy Klobuchar poses for a portrait in Washington in January 2019. She has been in Congress since 2007. Hide Caption 1 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar was born in Plymouth, Minnesota, in 1960. Her mother, Rose, was a teacher, and her father, Jim, was a columnist for the Star Tribune, Minnesota's largest newspaper. Hide Caption 2 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar's first-grade photo, which she posted on Twitter in 2016. Hide Caption 3 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar was magna cum laude at Yale University, earning a bachelor's degree in political science. During college, she worked as an intern for Vice President Walter Mondale. In 1985, Klobuchar earned a law degree from the University of Chicago Law School. Hide Caption 4 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar married attorney and professor John Bessler in 1993. Hide Caption 5 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar holds her daughter, Abigail, in 1995. At the time, Klobuchar was a partner at a Minneapolis law firm. Hide Caption 6 of 37 Photos: Former presidential candidate Amy Klobuchar In 1998, Klobuchar was elected as the attorney of Minnesota's Hennepin County, which includes Minneapolis. She held that position until running for the US Senate. Hide Caption 7 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar celebrates with her husband and daughter after she was elected to the Senate in November 2006. Klobuchar won 58% of the vote to become the state's first-ever female senator. Hide Caption 8 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar participates in a ceremonial swearing-in with Vice President Dick Cheney in January 2007. Hide Caption 9 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar holds a toy train with lead paint as she testifies before a Senate subcommittee about toy safety standards in September 2007. Hide Caption 10 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar and Minnesota Gov. Tim Pawlenty survey homes that were destroyed by a storm in Hugo, Minnesota, in May 2008. Hide Caption 11 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar test-drives an electric vehicle to the US Capitol in June 2008. Hide Caption 12 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar talks on her cell phone as she rides the Capitol subway in July 2008. Hide Caption 13 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar, fifth from left, joins other female senators on stage at the Democratic National Convention in August 2008. Hide Caption 14 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar shows off her shoes to the press as she attends a White House dinner for German Chancellor Angela Merkel in June 2011. Hide Caption 15 of 37 Photos: Former presidential candidate Amy Klobuchar In March 2013, Klobuchar and US Sen. Al Franken attend a Senate hearing about a measure that would reinstate a ban on assault weapons. Hide Caption 16 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar gets on an elevator on Capitol Hill in October 2013. Hide Caption 17 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar delivers a keynote address to the BlueGreen Alliance Foundation's "Good Jobs, Green Jobs" conference in February 2014. Hide Caption 18 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar listens as Federal Reserve Chairwoman Janet Yellen testifies during a Senate hearing in May 2014. Hide Caption 19 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar, right, is joined by other female senators as she testifies during a Senate subcommittee hearing about violence and discrimination against women. Hide Caption 20 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar walks through Statuary Hall on her way back to the Senate following a joint meeting of Congress in April 2015. Hide Caption 21 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar appears on television during a hearing about the avian flu in July 2015. Hide Caption 22 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar and several of her Senate colleagues attend a photo shoot for National Seersucker Day in June 2016. Hide Caption 23 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar speaks at the Democratic National Convention in July 2016. Hide Caption 24 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar, right, talks with Sen. Debbie Stabenow in a Capitol Hill hallway in November 2016. Hide Caption 25 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar, in the lime-green jacket, joins two other US senators as they pose with Ukrainian service members in December 2016. To the right of Klobuchar are US Sen. John McCain, Ukrainian President Petro Poroshenko and US Sen. Lindsey Graham. Hide Caption 26 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar helps count Electoral College votes during a joint session of Congress in January 2016. Hide Caption 27 of 37 Photos: Former presidential candidate Amy Klobuchar During the inauguration of President Donald Trump, Klobuchar takes a selfie with, from left, US Sens. Chris Van Hollen, John McCain and Bernie Sanders. Hide Caption 28 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar and other members of the Senate Judiciary Committee attend a hearing about Supreme Court nominee Brett Kavanaugh in September 2018. Hide Caption 29 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar is joined by her husband and daughter as Vice President Mike Pence administers the Senate oath of office in January 2019. Klobuchar was just starting her third term in office. Hide Caption 30 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar, left, takes a photo with US Sen. Tammy Baldwin, center, and US Rep. Ilhan Omar before the State of the Union address in February 2019. Hide Caption 31 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar announces her presidential bid on a snowy day in Minneapolis in February 2019. Hide Caption 32 of 37 Photos: Former presidential candidate Amy Klobuchar A person sits with Klobuchar's memoir during a Klobuchar campaign stop in Albia, Iowa, in February 2019. Hide Caption 33 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar gestures during the first Democratic debates in June 2019. Hide Caption 34 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar speaks during an event in Hanover, New Hampshire, in February 2020. She finished third in the New Hampshire primary. It was her highest finish of the campaign. Hide Caption 35 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar endorses former Vice President Joe Biden in March 2020. Hide Caption 36 of 37 Photos: Former presidential candidate Amy Klobuchar Klobuchar pays her respects to George Floyd before a memorial service in Minneapolis in June 2020. Hide Caption 37 of 37

The same issues confronted Klobuchar in South Carolina, where the senator had just a handful of staffers permanently assigned to the state for much of 2019.

Klobuchar largely abandoned South Carolina in the week leading up to the primary, holding just one public event in the days before voting. She instead focused on a host of Super Tuesday states that her campaign believed would be friendlier to her.

The desertion was not missed by operatives in South Carolina.

"I'm dumbfounded," said Bakari Sellers, a former South Carolina Democratic legislator and CNN analyst. "I'm trying to figure out how she thinks she can be the nominee without the black vote. Pete Buttigieg has at least tried. Amy Klobuchar doesn't even try. It's evident. South Carolina voters are paying attention and are going to punish her in the primary."

Klobuchar, like all the other senators in the race, confronted the unexpected issue of having to be in Washington, DC, for the impeachment trial of the President, an obligation that kept the senator off the campaign trail days at a time in January.

"I never thought I wasn't going to be able to be back here all next week, never in my wildest dreams," Klobuchar said on the Sunday before the final week leading up to the Iowa caucuses. "But that is what my life is, and I am doing the right thing."

But the impact on Klobuchar was greater. Both Sanders and Massachusetts Sen. Elizabeth Warren had more national campaigns and better odds at winning in states beyond Iowa, even if they finished there. Klobuchar never enjoyed that kind of broad support.

Klobuchar entered the race in February 2019 at an outdoor event where the senator was blanketed in snow minutes after taking the stage. She cast herself as the product of working-class parents and pledged to take on issues like money in politics, climate change and election reform, all while leaning on her Minnesota roots.

The pitch worked consistently with at least one group: newspaper editorial boards. The senator touted the support of newspaper editorial boards throughout her run, including local papers in Iowa and an endorsement shared with Warren from the New York Times.

Unlike some candidates who tried to avoid getting too aggressive early in the race, the Minnesota native was one of the more direct candidates on the debate stage. The senator hit Warren for her litany of plans, telling voters that "the difference between a plan and a pipe dream is something that you can actually get done."

And some of the senator's most heated barbs were aimed at former South Bend, Indiana, Mayor Pete Buttigieg, who also recently exited the Democratic race. Klobuchar often went after Buttigieg for suggesting that experience in federal office was not the kind of experience needed in a president.

That acrimony reached a head days before the Nevada caucuses, where the two moderate candidates sparred aggressively.

When Buttigieg questioned Klobuchar's inability to name the president of Mexico in a recent interview, the senator shot back, "Are you trying to say that I'm dumb? Or are you mocking me here, Pete?" When the former mayor slammed her for voting for Trump's Customs and Border Patrol head, the senator dismissively said, "I wish everyone was as perfect as you, Pete," before touting the fact she was "in the arena."

But Klobuchar's aggressive style at that debate failed to pay off for her in Nevada.

Klobuchar leaves the race with a warning to Democrats, especially because the front-runners for the nomination are now two white men in their 70s. Klobuchar, at debates and events across the country, has highlighted the ability for women to win, pointing to the success of Democratic women in the 2018 midterms as proof.

"Look at the facts. Michigan has a woman governor right now and she beat a Republican -- Gretchen Whitmer. Kansas has a woman governor right now and she beat Kris Kobach," Klobuchar said at the CNN/Des Moines Register debate in Iowa. "You have to be competent to win and you have to know what you're doing."

The senator put a finer point on it: "You don't have to be the skinniest person in the room. You don't have to be the loudest person. You have to be competent."

This story has been updated with additional developments.