A challenge in the polls ... Republican presidential candidate Donald Trump is on the campaign trail. Picture: AP/Mark Humphrey Source: AP

Real estate magnate Trump has spent weeks dominating the Republican field, and has emerged as a credible White House candidate for many voters in Iowa, the first crucible of the 2016 nominating contest.

But with Carson’s stellar showing in a series of recent polls in Iowa and nationwide, and another survey showing Senator Bernie Sanders nipping at Democratic frontrunner Hillary Clinton’s heels, the second-place candidates are exhibiting considerable power on the campaign trail.

Watch out ... Billionaire Donald Trump could be overtaken in Iowa polls. Picture: AP/Richard Shiro Source: AP

Only one other candidate polls in double digits: another political novice — former Hewlett-Packard boss Carly Fiorina, with 10 per cent.

Senator Ted Cruz, Wisconsin Governor Scott Walker and former Florida governor Jeb Bush round out the top five.

“These results mark a significant shake-up in the leaderboard from Monmouth’s Iowa poll taken before the first (Republican presidential) debate” in early August, said Monmouth University Polling Institute director Patrick Murray.

At that point, Walker, of neighbouring Wisconsin, led the rankings. The calm-demeanored Carson’s favourability rating is sky-high at 81 per cent to six per cent, while Fiorina also shines at 67 to eight. Trump’s favourability is 52-33.

Political outsiders surge in the US presidential race ... Retired neurosurgeon Ben Carson has drawn even with frontrunning billionaire Donald Trump in a poll in early-voting Iowa. Picture: AFP/MANDEL NGAN/FILES Source: AFP

Bush by comparison is deep underwater at 32-51 per cent, according to Monmouth. Carson, 63, is aiming to become the first African American to win the Republican nomination.

He entered the nation’s political consciousness two years ago with a speech criticising President Barack Obama’s policies in front of Obama himself.

Carson, who despite a childhood of dire poverty in Detroit, rose to become one of the nation’s most respected medical professionals through his presidential campaign is seeking political “healing” and a renewed sense of compassion in America.

His campaign has been low-key compared to the braggadocious Trump, but Carson is seen as earning strong support from Christian conservatives, an important voting bloc in Iowa.

Finding fans ... Experts say Trump and Carsen are both finding traction because they don’t seem like politicians and there’s a demand for that at the moment. Picture: AP/Mel Evans Source: AP

“Trump and Carson, one bombastic and the other sometimes soft-spoken, could hardly be more different in their outward presentations,” according to respected pollster Ann Selzer, who released a Des Moines Register/Bloomberg poll of likely Iowa caucus-goers showing Trump leading with 23 per cent and Carson second with 18 per cent.

“Yet they’re both finding traction because they don’t seem like politicians and there’s a strong demand for that right now.” Cruz and Walker are tied for third with eight per cent in the Des Moines Register poll.

Bush and Senator Marco Rubio are next at six per cent, while Fiorina earned five per cent.

Shunning the political establishment is occurring to a degree on the Democratic side as well, according to the Register poll.

While results show Clinton ahead with 37 per cent, she has lost one third of her support since May, while the liberal Sanders is now just seven points behind in the Democratic nomination race, at 30 per cent.