US Republican presidential candidate Donald Trump speaks as (L-R) his wife Melania, daughter Ivanka and Ivanka's husband Jared Kushner listen, at a campaign rally on caucus day in Waterloo, Iowa February 1, 2016. [Photo/Agencies]

Iowans will begin choosing candidates at 7 p.m. CST (0100 GMT on Tuesday), with results expected within a few hours.

A win for Trump could validate an aggressive campaign that has alarmed the Republican establishment, dwarfed the efforts of many seasoned politicians and has been marked by controversies such as his calls for a temporary ban on Muslims entering the United States and for a wall along the Mexican border.

A loss would dent the campaign persona of "winner" that the real estate tycoon and former reality TV star has carefully cultivated, and create immense pressure for a better performance in the next nominating contests - in New Hampshire and South Carolina - later this month.