Beto O'Rourke, who entered the Democratic presidential contest on a wave of hype but slowly sank from contention as his performance failed to match the epic buildup, abruptly ended his candidacy Friday.
In a Medium post and a series of tweets on Friday afternoon, the former congressman from El Paso said: "Our campaign has always been about seeing clearly, speaking honestly, and acting decisively".
He ran for the U.S. Senate and lost to incumbent Ted Cruz in 2018.
O'Rourke told supporters in an email that he will not run for office next year.
But Mr O'Rourke, 47, struggled to replicate that model in the presidential primary, and both his polling and his fundraising dwindled significantly in recent months.
O'Rourke then lashed out, seeking to recapture the imagination of the progressive wing that animates much of today's Democratic Party.
Considered a media darling, Mr O'Rourke was seen as the breakout star of the midterm elections when he nearly defeated Texas Republican senator Ted Cruz in the traditionally GOP seat - falling just 3% short. But the enthusiasm quickly petered out, and O'Rourke struggled to raise money from more traditional sources.
Rather than sticking to his pragmatist bona fides, O'Rourke strayed to the left, sometimes even to the left of his Democratic rivals, proposing mandatory national buybacks for assault weapons and revoking the tax-exempt status of American churches that refuse to support same-sex marriage.
While explaining his decision to step back from the 2020 race, Mr O'Rourke urged supporters to "unify around a nominee".
Some presidential hopefuls can run unsuccessful campaigns and end up with an elevated stature among party faithful - with more political prospects than when they began. It was a strategy he later applied to his presidential bid following a racially motivated mass shooting in his hometown of El Paso, Texas, that claimed the lives of 22 people.
His position led to criticism from Democrats in Washington, including Senate Minority Leader Chuck Schumer of NY.
Although O'Rourke secured his spot in the September and October debates, he had yet to qualify for the fifth Democratic debate in November prior to ending his campaign. That means he will also not become a Senate candidate in Texas, despite pleas from some Democrats for him to take on Republican Sen.