(REUTERS) - US Republican presidential candidate Donald Trump asked a crowd of supporters on Sunday (Jan 24) if a protester wearing a turban, who had just been escorted out of his campaign venue, was "wearing one of those hats".

Two men, one in a red turban, unfurled a banner during Mr Trump's speech at a campaign stop in Muscatine, Iowa, that read "Stop the Hate".

Mr Trump made the "hat" remark as both men were being escorted out of the venue.

"He wasn't wearing one of those hats, was he? Was he wearing one of those?" Mr Trump asked the crowd, adding that the man "never will", although it wasn't clear what the White House hopeful meant by the second remark.

Mr Trump, the New York billionaire and former reality TV star who has been virtually impervious to attacks from his opponents, has been pushing the limits of his political rhetoric.

He has been criticised for some of his remarks about immigrants and Muslims, and just a day earlier he said that he "could stand in the middle of Fifth Avenue and shoot somebody and I wouldn't lose any voters".

Mr Trump has been a difficult target for criticism from his rivals because not all of his supporters are conservatives and many are most interested in his projection of strength, not where he stands on a particular issue.