WASHINGTON -- Once considered a favorite for the Republican presidential nomination, Wisconsin Gov. Scott Walker announced Monday that he is dropping out of the race, amid sinking poll numbers and fundraising concerns.

"I suspend my campaign immediately," he said at a press conference in Madison, Wisconsin.

Walker implored other candidates to consider exiting the race as well, in order to winnow the field.

"I encourage other candidates to do the same so that the voters can focus on a limited number of candidates who can offer a positive, conservative alternative to the current front-runner," he said, referring to real estate mogul Donald Trump . "This is fundamentally important to the future of the party, and more importantly, to the future of the country."

Walker also criticized the other GOP contenders for resorting to "personal attacks" and straying from the party's policy message.

"In the end, I want voters to be for something, not against someone," Walker said. "We need to get back to the basics of our party."

Walker informed GOP sources and campaign supporters of his decision earlier Monday, according to The New York Times . Campaign officials then confirmed the announcement to several other news outlets.