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Democrats and Republicans face tight race heading into Iowa

With just three weeks until Iowa, the days grow shorter, and the polls grow tighter.

Donald Trump has a three-point lead over Ted Cruz, but that's within the poll's margin of error.

Trump spent Monday questioning Cruz's legal standing to run for president, asserting his birth in Canada to an American mother leaves doubt.

"It's wrong to say it is a settled matter because it's absolutely not, it's not a settled matter. That means, you know a lot of people think you have to be born here," Trump said at rally in New Hampshire before a lethargic and relatively small midday crowd.

Trump continued planting doubts about Ted Cruz's citizenship, saying GOP voters will render the final verdict.

"I mean it's one of those little decisions. I'm sure Ted is thrilled that I'm helping him out but I am. I mean I am. I mean he's gotta go and he's got to fix it."

Cruz has dismissed Trump's suggestions, saying he's "never breathed a breath of air on this planet when I wasn't an American citizen by virtue of being born."

A new poll shows Trump with a commanding lead in New Hampshire, with John Kasich, Marco Rubio and Cruz in a virtual tie for second. But Trump's camp fears a Cruz victory in Iowa would catapult him ahead in New Hampshire.

Rubio is also looking over his shoulder at Chris Christie, who is trailing him by just four points in New Hampshire. On Monday, Rubio attacked Christie on key conservative issues.

"Our next president cannot be someone who supports Common Core, who supports gun control, who's personally contributed to Planned Parenthood. These are things we need to be reversing," said Rubio.

Christie defended his record on "Face the Nation."

"Marco himself has said that I was a conservative reformer in New Jersey. So here's the thing. I'm not going to spend my time talking about Marco Rubio."

On the Democratic side, Hillary Clinton normally takes aim at her Republican rivals. But in Waterloo, Iowa on Monday, she focused on Bernie Sanders and his proposals for new spending.

"There's no way if you do the arithmetic, how to pay for what he's proposing without taxes on the middle class," Clinton argued.

The change in course was no coincidence. The NBC/Marist poll shows Sanders coming on fast in Iowa, and even outperforming Clinton in the general election.

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In a theoretical match up with Donald Trump, Sanders leads in Iowa by 13 points. Clinton leads by eight.

A match up with Ted Cruz shows Sanders leading by five points, while Clinton trails by four.

Sanders told Iowa's largest paper that Clinton's new attacks on him show she's, quote, "nervous" and "panicky," but he's doing the exact same thing.

"You know these polls go up, they go down," Clinton said Sunday on "Face the Nation."

The former first lady downplayed both the poll numbers and Trump's recent comments about her husband's infidelities in the 1990s.

"They can say whatever they want. More power to them. I think it's a dead end, blind alley for them, but let them go."

Clinton leads in later states like South Carolina by up to 40 points, but that could change if she under performs in the first two contests.