With new polls showing Ted Cruz still leading Donald Trump in Iowa, Trump used a rally in the Hawkeye State on Saturday night to launch an extended attack on Cruz, again zeroing in on how Cruz was born in Canada, raising questions about whether that disqualifies the Texas Senator to be President of the United States.

"If he ever got the nomination, you know Democrats would bring a major suit," Trump said in Clear Lake, Iowa, as Trump repeatedly told the crowd that Cruz was born north of the border.

"He was born in Canada, I guess his parents voted in Canada," Trump added in what was an almost four minute "birther" attack on Cruz.

"If it's a problem," Trump said, "you got to work it out," as Trump all but demanded that Cruz go to court, and get a judge to declare that he is eligible to run for President.

The extended Trump attack on Cruz did not go unnoticed by Cruz supporters, who pointed out that Trump is not emphasizing any differences on issues - just the question of Cruz's birthplace.

So far, the conservative talk radio world - much of which has been very vocal in support of Trump and his drive against the GOP establishment - has refused to join Trump in these attacks on Cruz, arguing that Cruz is a legitimate candidate.

Meanwhile, a focus group of Iowa voters conducted by well known pollster Frank Luntz found that Trump's "birther" attack on Cruz is not working.

The Des Moines Register reported that voters were aware of Trump's attacks, but only a few said it was big concern.

It's possible the issue might come up later this week, when Republicans meet for their next debate on Thursday night in South Carolina.

The Iowa Caucus is just three weeks away, on February 1.