Media playback is unsupported on your device Media caption Scott Walker: "I'm helping to clear the field"

Once considered a front-runner, Wisconsin Governor Scott Walker has decided to end his campaign for US president amid dwindling contributions and plummeting poll numbers.

Unlike some rivals, the Republican had a large and costly campaign operation.

He topped polls in the beginning of 2015 but lost popularity as Donald Trump began to dominate the race.

A recent CNN poll shows he had support of less than one half of one percent of Republican primary voters.

"Today I believe that I am being called to lead by helping to clear the field in this race so a positive conservative message can rise to the top of the field," Mr Walker said on Monday.

He encouraged other underperforming Republican presidential candidates to also exit the race so voters will have "a positive conservative alternative" to Mr Trump.

Some analysts believe Mr Trump, who is at odds with Republican leadership on key issues like tax policy, has benefited as voters have split over a field of more than a dozen candidates.

Analysis: Anthony Zurcher, BBC North America reporter

Image copyright Getty Images Image caption Mr Walker's poll numbers plummeted after the first two Republican debates

One of the mighty has fallen. Earlier this year Wisconsin Governor Scott Walker was considered a top-tier presidential contender with the potential to unite the establishment and grass-roots wings of the Republican Party.

Instead, the governor is heading back to Wisconsin months before Iowa - a nearby state he once was a favourite to win - holds its first-in-the-nation caucuses.

The post-mortem reports on the Walker campaign will likely focus on the candidate's numerous public missteps after he shot to prominence in early 2015 - such as comparing his state's union protestors to Islamic State militants and taking no position on the theory of evolution.

His support didn't crater until his lacklustre performance in the first two Republican debates, however, when he seemed unwilling or unable to vie for attention on the crowded stage.

Reports abounded that his donors were starting to look elsewhere - to competitors like former business executive Carly Fiorina, Florida Senator Marco Rubio and retired neurosurgeon Ben Carson. Mr Walker was likely facing a cold truth. He had built the infrastructure for a national campaign, but the money to run it was about to dry up.

Image copyright Getty Images Image caption Organised labour groups were strongly opposed to Mr Walker's policies

Mr Walker rose to national prominence for his opposition to organised labour groups.

As Wisconsin governor, he sparked large protests after rolling back protections for public sector unions.

He overcame a recall attempt in 2012 by his political opponents and was later re-elected as governor in 2014. Those successes were often touted as central to his appeal as a presidential candidate.

He rocketed to first place in early polling after a rousing speech touting independence from the government to the Iowa Freedom Summit in January.

However, Mr Walker, an elected official since 1993, struggled to gain traction as Republican voters gravitated toward political outsiders like Mr Trump and retired neurosurgeon Ben Carson.

Shifting positions

He was also criticised for shifting positions on several key issues including immigration reform.

He originally said he wanted to make a pathway for undocumented immigrants to gain legal status and then said he only supported securing the border.

Mr Walker joins Rick Perry, the former governor of Texas who also recently dropped out of the race.

Fifteen Republican candidates remain.