Beto O'Rourke has announced that he is dropping out of the race to become the Democrats' 2020 presidential contender.
Mr O'Rourke, a 47-year-old former congressman representing Texas, said it had become clear his campaign did not have the resources to continue to seek the party's nomination.
"My service to the country will not be as a candidate or as the nominee," he wrote on Twitter.
"Acknowledging this now is in the best interests of those in the campaign; it is in the best interests of this party as we seek to unify around a nominee; and it is in the best interests of the country."
Donald Trump immediately mocked him on Twitter, tweeting gleefully: "Oh no, Beto just dropped out of the race for president despite him saying he was 'born for this'. I don't think so!"
Mr O'Rourke had been struggling to break through a crowded Democratic field.
He entered the race in March with buzz from his narrow 2018 Senate loss to Republican incumbent Ted Cruz in Texas.
But as the excitement over his candidacy began to fade, Mr O'Rourke was forced to stage a "reintroduction" of his campaign to reinvigorate it.
After a mass shooting in his hometown of El Paso, Texas, he began to center his campaign on gun violence prevention.
He failed to inspire confidence during the series of the Democrat debates, and performed remarkably poorly in the polls.
His decision to pull out does come as something of a surprise, however, given that much lesser-known faces still remain in the contest.