Wisconsin Gov. Scott Walker (R) Getty Wisconsin Gov. Scott Walker (R) is out.

Once seen as a potential dark horse in the GOP presidential primary, Walker made his exit on Monday evening.

Walker announced his exit at a hastily-scheduled press conference. He attributed the decision to a desire to help voters "focus" on more "positive" alternatives to the current Republican frontrunner, Donald Trump.

"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," Walker said. "With this in mind, I will suspend my campaign immediately. I encourage other Republican presidential candidates to consider doing the same so that the voters can focus on a limited number of candidates who can offer a positive, conservative alternative to the current frontrunner."

The New York Times was first to report the news shortly before Walker's announcement. A major Walker donor subsequently confirmed his intention to drop out to Business Insider.

The Times' Patrick Healy and Alexander Burns wrote that Walker "concluded he no longer has a path to the Republican presidential nomination and plans to drop out of the 2016 campaign."

Walker's campaign has not responded to multiple requests for comment from Business Insider. Keith Gilkes, the executive director of Unintimidated PAC, a political action committee that supported Walker, released a statement just after the announcement. In it, Gilkes credited Walker with a string of "unprecedented victories" for the "conservative movement" in his home state.

"Governor Walker's decision to gracefully exit the race was no doubt tremendously difficult for him and his family, but it is a testament to the type of man and leader that he is," Gilkes said.

Walker is perhaps best known for his battles with public-sector unions in Wisconsin. Earlier this year, he raised eyebrows with a surprisingly strong performance in a poll of voters in Iowa, an influential early-voting state.

More recently, he has polled at the bottom of the Republican field, and he received little screen time in the GOP debate last Wednesday. Walker, who was third in national polling of the Republican primary in July, received less than one-half of 1% in a CNN poll released this weekend.

Walker is the second major Republican candidate to bow out of the race, joining former Texas Gov. Rick Perry.