IOWA (REUTERS) - US Republican presidential candidate Donald Trump brought out his family at a rally in Council Bluffs, Iowa, on Sunday (Jan 31), one day before the first nominating contest.

"He will be unbelievable - the best deal-maker, the best master negotiator," the billionaire businessman's wife Melania told the crowd.

Also appearing on-stage was Mr Trump's daughter Ivanka, who is 8.5 months pregnant, and son-in-law Jared Kushner.

"Politically, wouldn't it be great if she had her baby in Iowa?" Mr Trump joked, referring to the doctors who had advised Ivanka not to travel. "That would guarantee me victory, right?"

A Des Moines Register/Bloomberg News poll of Iowa voters released on Saturday showed Mr Trump in the lead with 28 per cent, Mr Ted Cruz second at 23 per cent, and Mr Mario Rubio in third with 15 per cent.

How Iowa will vote, however, is notoriously difficult to predict since it is never quite certain how many people will show up to caucus.

"I have no doubt that [my father] will make America great again, but we also need all of you on Monday night to help us do that," Ivanka told rally-goers. "He will over deliver."