The 47-year-old Wisconsin governor tried to appeal to religious conservatives, tea party members and traditional Republican supporters during his run. He campaigned on a Harley-Davidson motorcycle and was applauded for a well-received speech among conservatives in Iowa earlier this year.

Despite once being considered a favorite for the 2016 nomination, Walker saw dwindling support in recent polls, due in part to the success of billionaire businessman Donald Trump.

Critics have also accused Walker of being unable to clearly state his position on several issues, including a decision on birthright citizenship and showing interest in building a wall between the US and Canada.

Walker's rise to national prominence began in 2011 when he moved to restrict labor unions in his state and forced workers to pay more into pensions and health plans. He even survived a recall election, earning him support from conservative circles around the US.

Considered a Democratic stronghold, Wisconsin supported President Barack Obama's reelection in 2012 by nearly 7 percentage points.

Walker is the second Republican to pull out of the race for the Republican party nomination in 2016. Former Texas Governor Rick Perry withdrew on September 11.

Carson under fire

Meanwhile, Republican candidate and neurosurgeon Ben Carson was the center of debate on Monday after a weekend interview in which he said he saw Islam as being incompatible with the position of head of state.

'I would not advocate that we put a Muslim in charge of this nation," Carson said Sunday, in an interview with broadcaster NBC.

Several US politicians and organizations spoke out against Carson's comment on Twitter, including former Secretary of State and Democratic front-runner Hillary Clinton and Senate Minority Leader Harry Reid.

Carson has yet to comment amid the backlash over his statement, but his campaign reported increased fundraising and 100,000 new Facebook friends in the last 24 hours.

mg/cmk (Reuters, dpa, AP)