Trump puts Cruz off balance by fanning ‘birther’ controversy
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Donald TrumpDonald TrumpThe Memo: Searching for firm footing as Trump Era begins 
Democrats vie for chance to take on Trump as California governor A voice for GOP women in DC MORE thinks he has found Ted CruzTed CruzTrump's America: Businessmen in, bureaucrats out When Trump says 'Make America Great Again,' he means it Booker is taking orders from corporate pharmaceuticals MORE’s kryptonite.

The Texas senator has been put on the defensive and off message since Trump began raising questions about his rival’s eligibility to serve as president. It’s an issue that resonates with GOP base voters, if few others.

Trump, the front-runner for the Republican nomination, hit Cruz, who is in second place in national polls, again on Monday, warning that Democrats would sue if the Tea Party favorite were the party’s -standard-bearer in the fall.

“Whether you like it or not, Ted has to figure it out. Because we can’t be having a nominee — if he got the nod, I think I’m going to win very solidly, if you want to know the truth — but, if you get the nomination, you can’t have the person who gets the nomination be sued,” Trump told supporters at a rally in Windham, N.H.   

Trump’s focus on eligibility seems to be gaining more traction than his attacks on other issues, such as Cruz’s stance on ethanol or his religious convictions.

Cruz was born in Canada to an American mother, a scenario most legal experts have long agreed meets the Constitution’s requirement to be a “natural born citizen.” 

But Trump’s salvos have forced Cruz to answer questions on his eligibility to serve in the Oval Office instead of framing the debate on more favorable terms.

“This is Trump’s way of steering the discussion in this race,” said Craig Robinson, a Republican strategist based in Iowa. “The reason I think this is actually a pretty smart tactic is because it goes to the heart of the base of Ted Cruz. Anything that creates doubt among your base supporters can be problematic.”

A new poll by Quinnipiac University shows Trump has retaken the lead from Cruz in Iowa, home to the first contest on the path to the GOP nomination.

The survey shows Trump ahead of Cruz by 2 percentage points, topping the GOP pack with 31 percent.

It’s only the third poll of the past 11 in Iowa showing Trump ahead of Cruz, according to aggregation by -RealClearPolitics.

Robinson noted that many of Cruz’s supporters — self-identified conservative males — rallied around Trump when he questioned President Obama’s eligibility to serve as commander in chief four years ago.

When he was mulling a White House bid ahead of the 2012 election, Trump raised doubts about whether Obama was born in the United States and even claimed to have sent investigators to the president’s home state of Hawaii to dig into the matter.

Obama put the issue to rest by releasing the long-form version of his birth certificate but not after it caused a public uproar and endeared Trump to the president’s conservative critics.

GOP strategists say wielding the same issue against Cruz is not likely to have legs but claim it has thrown him off balance for the moment. 

“It’s a baseless allegation, and it may be an interesting thumb-sucker for constitutional lawyers, but I think it’s settled law, and I don’t think there’s any question about that,” said Tom Rath, a longtime Republican strategist based in New Hampshire.

“Having said that, I think one of the things that’s clear is that the Trump campaign is really good at keeping the ball moving. Every time you think you’ve got them on an issue and you drill down on the issue, the next day they come up with something different to talk about. That’s pretty effective,” he added. “Clearly at the moment they see a problem in Iowa with Cruz.”

Other lines of attack have been less effective. 

Conservative radio host Mark Levin slammed Trump for “pandering” to Iowa voters last month after he bashed Cruz for not supporting the Renewable Fuel Standard, a government program that benefits ethanol producers. Trump has also cast doubt on the sincerity of Cruz’s religious views, noting “not a lot of evangelicals come out of Cuba.”

“Trump has tried different tacks. At the last Republican debate, he was attacking Cruz on ethanol from the left, saying he should be supporting ethanol,” said Amanda Carpenter, who formerly served as Cruz’s communications director. “Talk radio didn’t like Trump attacking Cruz from the left, and he didn’t really pursue it anymore, and so he went after the birther theme because he’s trying different themes.”

Trump has seen support over the issue from across the political spectrum, perhaps more a sign of Cruz’s broad range of enemies than anything else.  

Citing liberal Harvard law professor Laurence Tribe, Trump on Monday warned that Cruz might be barred constitutionally from serving as president.

“Ted Cruz has a problem. Because the question is: Is he an actual born citizen?” Trump said Monday.

Trump also raised it at a rally in Reno, Nev., Sunday, asking a crowded ballroom of supporters, “Is he a natural born citizen?”

“Honestly, we don’t know. Who the hell knows?” he said in response to his own question.

The Constitution states a candidate for president must be 35 years old, a resident of the country for 14 years and a natural born citizen. What the third qualification means has become a subject of fierce debate.

Trump received a boost Monday when Iowa’s Republican governor, Terry Branstad, joined in the fray, declaring the question of Cruz’s citizenship fair game.

“When you run for president of the United States, any question is fair game. So let the people decide,” he told reporters.

Cruz’s rivals, sensing weakness, have piled on.

Sen. Rand PaulRand PaulTrump's CIA chief clears Senate Overnight Defense: Trump nominates Air Force secretary | Senate clears CIA director | Details on first drone strike under Trump Dems blast Trump plans for deep spending cuts MORE (R-Ky.), who is also running for president, predicted on Sunday that Democrats would challenge Cruz’s eligibility to serve as president if he were to win the nomination. Rep. Alan GraysonAlan GraysonWhy Republicans took aim at an ethics watchdog Could bipartisanship rise with Trump government? Schumer under pressure to add Sanders to leadership team MORE (D-Fla.), who is running for the Senate, has threatened to do just that.

Sen. John McCainJohn McCainSenate panel votes to confirm Tillerson Overnight Defense: Trump nominates Air Force secretary | Senate clears CIA director | Details on first drone strike under Trump McCain: Trump's withdrawal from TPP a 'serious mistake' MORE (R-Ariz.), one of Cruz’s most outspoken critics in the upper chamber, has also questioned his eligibility. 

McCain, who served as the GOP presidential nominee in 2008, was born in the Panama Canal Zone, but his eligibility was not a hotly debated issue that year.