Donald Trump showed no cracks in the armour of his campaign to return to the White House after scoring an overwhelming victory in Iowa over his closest Republican rivals, prevailing in all but one of the Midwestern state's 99 counties.

The result was widely expected, but it confirmed the tight grip Trump has retained over most voters in his party, who remain devoted to him despite the criminal charges he faces in courts across the US and yearn to see him back in office.