Donald Trump in Iowa
Donald Trump greets supporters at a campaign rally in Muscataine, Iowa.Reuters

Front-running Republican candidate Donald Trump was sweeping all before him in Iowa until a church pastor appeared to chastise him in front of her congregation during a service that Trump was attending.

"Jesus is teaching us today that he has come for those who are outside of the church," said the Rev Pamela Saturnia, pastor of the town of Muscataine's First Presbyterian Church, while Trump sat silently. She preached a message of healing and acceptance for "those who are the most unloved, the most discriminated against, the most forgotten in our community and in our world," including "Syrian refugees" and "Mexican migrants," the Associated Press reported.

It appeared to be a direct criticism aimed at the candidate who has called for a ban on all Muslim immigrants and has branded Mexicans who have come to America "rapists."

"I don't know if that was aimed at me ... perhaps," Trump said after the service, where he placed $100 (£70) in the collection basket.

The religious vote is critical in the Iowa caucuses on 1 February, the first contest in the nation in the race for the Democratic and GOP presidential nominee. Trump's closest rival is Ted Cruz, a pastor's son who has major support from evangelicals, though Sarah Palin's recent endorsement has lured others to Trump's side.

Trump has been pulling out the stops in Iowa. So devoted is he to winning that he spent a night in a place he may never have before visited: A Holiday Inn Express, a far cry from his his usual weekend digs in his estate in Palm Beach, Florida.

"It's crunch time, folks," Trump told the New York Times before a campaign rally . "I mean, I want to win Iowa. I really want to win it."

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Trump has continued to create controversy, including saying he could "shoot somebody" and it wouldn't hurt his poll numbers, and he seemed to be right. His lead is growing in New Hampshire, the next state after Iowa to hold a primary election.

Nor does criticism seem to hurt him. In a dig at Trump, former US Secretary of Defense and CIA Director Robert Gates lambasted the unsophisticated level of dialogue in the campaign. "I think middle school kids would be embarrassed by the level of dialogue going on in the national campaign," he said in a talk before the Council of Foreign Relations.

"It's clear they don't know what they're talking about," Gates added, citing some of the more outrageous remarks about making "the sand glow," and "I would bomb the s**t out of them," which he reminded the audience was said by the "the leading candidate," meaning Trump.

"This is not a particularly sophisticated analysis of the challenge that we face," Gates said in a speech. "The worrying thing is [if] they actually believe what they are saying, and if that is the case we really are in trouble."