Scott Walker has halted his 2016 White House bid, warning the Republican race has become too nasty.
He has also called on some of his party rivals to do the same - citing an urgent need to “clear the field” to help defeat front-runner Donald Trump.
The announcement marked a dramatic fall for Mr Walker, 47, who was struggling to generate fund raising and enthusiasm after surging into the race’s top tier earlier in the year.
He will return to his job in Wisconsin as governor, where his term runs until 2018.
“Today, I believe that I am being called to lead by helping to clear the field in this race so that a positive conservative message can rise to the top of the field. With that in mind, I will suspend my campaign immediately,” Mr Walker said in Madison, Wisconsin.
“I encourage other Republican presidential candidates to consider doing the same so the voters can focus on a limited number of candidates who can offer a positive conservative alternative to the current front-runner.
“This is fundamentally important to the future of the party and more importantly to the future of our country.”
One of the last Republicans to enter the race, Mr Walker joined former Texas governor Rick Perry as one of the first to leave it, unable to adjust to the popularity of billionaire property mogul Mr Trump or break out in either of the party’s first two debates.
Both candidates warned of Mr Trump’s influence on the Republican Party as they exited, although neither mentioned him by name.
“Sadly, the debate taking place in the Republican party today is not focused on that optimistic view of America. Instead, it has drifted into personal attacks,” Mr Walker said.
“In the end, I believe that voters want to be for something and not against someone. Instead of talking about how bad things are, we want to hear about how we can make them better for everyone.”
Mr Walker’s fall was in many ways more dramatic than Mr Perry’s.
He was thought to be a leader in the crowded field for much of the year and built a massive national organisation with paid staff spread across the country that dwarfed many of his rivals in scale and scope.
“I’m not sure what went wrong,” said Iowa state senator Mark Costello, who endorsed Mr Walker earlier this year. “I think all the more provocative statements some of the candidates made got them more press.
“I don’t think he made any really big mistakes but people lost enthusiasm.”
Mr Walker tried to appeal to religious conservatives, tea party conservatives and the more traditional Republican base. He tried to cast himself as an unintimidated conservative fighter who had a record of victories in a state that has not voted Republican for president since 1984.
Like Mr Perry, however, Mr Walker found little room for such a message in a race dominated by Mr Trump.
Mr Trump, leading most polls, said he got to know Mr Walker well, saying in a tweet “he’s a very nice person and has a great future”.
Mr Walker came to the race having won election in Wisconsin three times in four years, and having gained a national following among donors and conservatives by successfully pushing his state to strip union bargaining rights from its public workers.