The endorsements came shortly after Klobuchar and Buttigieg pulled their names from contention while O'Rouke dropped out of the race in November.
Buttigieg -- the former mayor of South Bend, Ind. -- stood alongside the former vice president as he said throwing his support behind Biden would help to fulfill the goal of his campaign to rally the country together to defeat President Donald Trump.
"That was always a goal that was much bigger than me becoming president. And it is in the name of that very same goal that I am delighted to endorse and support Joe Biden for president of the United States," he said.
Buttigieg began the primary season with a narrow win in the Iowa caucus over Bernie Sanders, which he followed up with a second-place finish behind the Vermont senator in New Hampshire. But on Sunday he said that his campaign's "path has narrowed to a close."
Amy Klobuchar, a three-term Minnesota senator who sought to unite the Democratic Party behind a pragmatic centrism, also dropped out of the race on Monday and offered Biden her endorsement one day before the crucial Super Tuesday primaries.
"He 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," she said.
She became the third Democratic presidential hopeful to drop out of the race in two days, following Buttigieg on Sunday and billionaire activist Tom Steyer earlier Monday.
The 59-year-old Klobuchar entered the race in February 2019 during a snowstorm in Minneapolis. Well-liked in her home state, where she has rolled to three consecutive lopsided Senate victories, she faced significant competition in the moderate lane from Biden and Buttigieg.
She pinned much of her hopes on neighboring Iowa, spending considerable time and resources in the state, where she was the only Democratic candidate to visit all 99 of its counties. Klobuchar, however, was disappointed with a fifth-place finish there.
She regained some momentum with a memorable performance in the Democratic debate before the New Hampshire primary, notching a third-place finish. But she could not translate that bump into a lasting trend, coming in sixth in both the Nevada caucuses and the South Carolina primary.
And then in the final minutes of the rally, Biden called O'Rourke to the stage.
The former Texas House representative told the crowd he will be casting his ballot Tuesday for the former vice president because "we need somebody who can beat Donald Trump."
Trump, O'Rourke said, poses an existential threat to the United States and Biden is his antithesis.
"At a time that this country is so polarized, so deeply divided, we need somebody who can bring us together and heal us," O'Rourke said. "We need somebody who can re-establish the moral authority of the United States. We need somebody who will fight for democracy here and abroad because democracy is under attack here and aboard. We need Joe Biden."
The 47-year-old Texan's endorsement of Biden comes months after he pulled his name from the presidential race in November, stating he lacked "the means" to continue.
Though qualifying for the four Democratic debates prior to dropping out, O'Rourke was trailing behind the poll requirements to gain admittance to the fifth match and was dragging behind in fundraising.
He served three terms in the House representing Texas' 16th Congressional District and had said after dropping out of the presidential race that he'd back whichever Democratic candidate is named the party's nominee.