War breaks out between Donald Trump and Ted Cruz

The shadow rivalry between real-estate mogul Donald Trump and Sen. Ted Cruz (R-Texas) broke out into the open this week, with the two presidential candidates taking their sharpest jabs yet at each other.

On Wednesday morning, Trump even declared that "there is no way that Ted Cruz can continue running in the Republican primary" because of the senator's birthplace in Canada.

"Is he allowed to run for president? If he wins, is he allowed to do it?" the Republican front-runner mused at a campaign rally the day before in Cedar Falls, Iowa.

More than any other candidate, Cruz has painstakingly avoided criticizing Trump. But that has now started to change.

Throughout the day Tuesday, Cruz dropped a number of new attack lines against Trump.

Cruz claimed that supporters of former Secretary of State Hillary Clinton were validating Trump's Canada argument because they think the business mogul would be easier for her to dispatch in the general election.

"And it may be driven by the fact that the polling right now shows Donald loses to Hillary - and loses by a pretty big margin. But I beat Hillary. And I think that's got the Hillary folks a little bit concerned. And so they're doing everything they can to amplify Donald's attacks," Cruz told radio host Howie Carr.

Trump has repeatedly questioned Cruz's eligibility to be president, though most legal experts believe the senator's American mother means he meets the constitutional requirement that the president be "natural-born" citizens.

Cruz also told Carr that Trump "embodies New York values." That came after apparent supporters of Cruz ran a poll in Iowa testing almost the exact same attack line against Trump, according to a Tuesday RealClearPolitics report.

Fox News host Megyn Kelly confronted Cruz about that reported message-testing poll during her interview with Cruz on Tuesday evening. Cruz suggested that he had no knowledge of the poll.

"I have no idea what calls are made to whom," he said.

"It sounds like you have may have had something to do with that message-testing, senator," Kelly said.

Cruz deflected by simply stating that his surge in the Republican primary had apparently "rattled" Trump.

"More and more observers are saying this is coming down to a two-man race between Donald and me," Cruz told Kelly. "And what we're seeing is conservatives are uniting behind our campaign, both in Iowa and New Hampshire, and all across the country. And that seems to have really rattled Donald. He's begun now every speech for a week with extended attacks directed at me."

Cruz said he wouldn't "reciprocate" Trump's attacks, but then noted yet again that Trump has been citing a prominent left-leaning legal scholar to question Cruz's eligibility.

"It starts to make you wonder, 'Gosh, why are Hillary Clinton's supporters backing Donald Trump's attacks?'" Cruz asked. "Maybe it has something to do with the fact that Hillary, in the national polling, beats Donald Trump badly. But I'm beating Hillary in the national polling."

Trump then appeared to dispute Cruz's claim about the national polls:

In another Tuesday interview, with conservative radio host Hugh Hewitt, Cruz also suggested that Trump didn't have much knowledge of US nuclear policy. Cruz was referencing last month's debate, in which Trump appeared to dodge a question about the US system of basing nuclear weapons on land, aircraft, and submarines.

"It is certainly relevant to voters," Cruz told Hewitt of the nuclear triad. "Does a potential commander in chief know what the nuclear triad is, much less is he or she prepared and able to strengthen it and keep this country safe?"

Cruz further took an apparent shot at Trump for saying last summer that he gets military advice from "the shows." Cruz said it's also relevant for voters to consider know how to defeat terrorism, "not just based on what's said on Sunday shows on TV, but actually understanding the nature of the threat."

For his part, Trump has far more aggressively dished out attacks at Cruz. At rallies, Trump has repeatedly brought up Cruz's Cuban ancestry and attacked him for being supposedly being too close to the oil industry.

And almost every day since last week, Trump has questioned Cruz's eligibility to serve as president: