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A highly-anticipated new poll shows a close race between Ted Cruz and Donald Trump in Iowa, with Marco Rubio and Ben Carson fighting for a distant third place as the caucus looms less than three weeks away.

According to a new Des Moines Register/Bloomberg poll, considered the gold standard of polling for the first caucus state, Cruz is the first choice of 25 percent of GOP caucus-goers, compared to 22 percent for Trump.

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Rubio receives 12 percent support, while Carson - who once led the poll back in October - gets 11 percent.

All other candidates languish in single digits; Rand Paul (five percent), Jeb Bush (four percent), Mike Huckabee (three percent), Chris Christie (three percent), Carly Fiorina (two percent), John Kasich (two percent) and Rick Santorum (one percent).

Cruz, whose eligibility to serve as president has been vocally questioned by Trump and whose stances on immigration and ethanol have also been under attack, has slipped six points since the last Des Moines Register/Bloomberg poll in early December.

But questions about Cruz's Canadian birthplace don't seem to be phasing most caucus-goers. Eighty-three percent say that they are not bothered that Cruz was born outside the United States, with just 15 percent disagreeing.

An NBC News/Wall Street Journal/Marist poll of Iowa released Sunday also showed a tight race between Trump and Cruz, with the Texas senator leading Trump 28 percent to 24 percent among likely caucus-goers, within the poll's margin of error.

The survey, conducted Jan. 7-10, included 500 likely Republican caucus participants. It has a margin of error of plus or minus 4.4 percentage points.