Pete Buttigieg, the former Indiana mayor who made an ambitious run for president, has announced he is ending his campaign for the White House.
The 38-year-old became the first openly gay presidential candidate from a major party when he announced he was running for the Democratic nomination.
But despite a successful start, his campaign lost momentum in recent weeks.
His decision to drop out comes ahead of a key day on Tuesday in the Democratic race to take on Trump.
Fourteen states will vote on Super Tuesday, by the end of which staunch left-winger Bernie Sanders could have an unbeatable lead and be a step closer to the nomination.
His departure leaves six Democrats still in the running - Joe Biden, Mr Sanders, Michael Bloomberg, Elizabeth Warren, Amy Klobuchar and Tulsi Gabbard.
Speaking to supporters in his hometown of South Bend, Indiana, Mr Buttigieg stressed the values he said his campaign had hoped to promote.
"And so we must recognise that at this point in the race, the best way to keep faith with those goals and ideals is to step aside and help bring our party and our nation together," he said. "So tonight I am making the difficult decision to suspend my campaign for the presidency."
He pledged he would do "everything in my power" to ensure a Democratic win in November's election.
Mr Buttigieg surprisingly, and narrowly, won the first event of the primary season, the caucuses in Iowa on 3 February. But he failed to repeat that success and win the delegates needed to make him the front- runner and later confirm his nomination. He finished a distant fourth in South Carolina on Saturday.
Mr Biden praised Mr Buttigieg's campaigning effort in a tweet.
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Mr Buttigieg's husband, Chasten, also addressed crowds of supporters in South Bend, saying: "About a year and a half ago, my husband came home from work and told me - well he asked me: 'What do you think about running for president?' And I laughed! Not at him, but at life.
"Life gave me some interesting experiences, on my way to find Pete. After falling in love with Pete, Pete got me to believe in myself... and I told Pete to run [for office] because I knew there were other kids sitting out there in this country who needed to believe in themselves, too."
How Buttigieg set his sights on the White House
It was one of the least likely runs for the presidency in years: Mr Buttigieg's only political experience had been as the centrist mayor of the 306th largest city in the US, South Bend, Indiana between 2012 and January this year.
Before then, he had served as a US Navy intelligence officer and in the Afghan war.
He was the first millennial to run for the White House, and would have been the youngest president to take office had he won.
The son of a Maltese immigrant, he had long been rumoured as a possible candidate in the 2020 election. But after announcing his candidacy in April last year, he was able to break through a crowded group to become one of the most recognisable faces in the campaign.
In that time, he raised more than $82m (£64m), according to the Federal Election Commission, one of the highest totals of all the candidates.
In recent weeks, his sexuality had been highlighted as an issue by critics - conservative radio host Rush Limbaugh asked if voters would pick a man "kissing his husband on stage".
Mr Buttigieg - pronounced Boot-Edge-Edge - came out as gay aged 33 and married his husband Chasten in June 2018.
He also struggled to build support among African-American voters, a point emphasised by his poor showing in South Carolina.
His record as mayor came under fire among minority voters - he had fired South Bend's first African American police chief and was criticised over how he handled the case of a white police officer who shot dead a black man last year.
How this news will help Biden
Just over a week ago, Pete Buttigieg had a lead in Democratic convention delegates. Now the former South Bend mayor is out of the race.
That's how quickly fortunes can change in presidential politics.
In the end, Buttigieg simply couldn't turn his early success into a national wave of support. While he thrived in the months of retail campaigning in the first two small - and exceedingly white - states, the Nevada and South Carolina results amply demonstrated that he was never going to catch on with the more diverse national Democratic electorate.
It's still a bit of a shock that Buttigieg pulled the plug rather than compete on Super Tuesday. By doing so, however, he avoids the potential for more defeats and helps Joe Biden, who will certainly appreciate the mayor's move to clear the field for him to better challenge Bernie Sanders one-on-one.
In the end, Buttigieg made an astounding effort, coming from nowhere to seriously challenge for the nomination, winning the Iowa caucuses and raising massive amounts of campaign cash. It's a remarkable accomplishment for the first openly gay presidential candidate with a difficult-to-pronounce name who was half the age of his closest presidential rivals.
His campaign will be one for the history books.
What happens next?
Super Tuesday is the most important date in the race to pick the nominee.
Democrats in 14 states will vote (as well as American Samoa and Democrats Abroad). A massive 1,357 delegates will be distributed - almost a third of all those available through the entire primary season. The two most populous states, California and Texas, will be among those voting.
The entire picture could change in one day. Or we could see Mr Sanders cement his lead as the front-runner - and even extend into a near-unbeatable lead.
This will also be the first time that New York billionaire Michael Bloomberg will be on the ballot. The performance of moderates like Mr Biden on Super Tuesday will be determined to some degree by how Mr Bloomberg fares.