Conservative icon, radio host and author of the No. 1 book in the country right now, Mark Levin said he’s sick of “birthers” after encountering a particularly “obnoxious” one at a hot, crowded book-signing event.

The man, whom Levin described as “disrespectful” toward him and the other fans, reportedly pointed at Levin and told him he was wrong in concluding that Sen. Ted Cruz, R-Texas, is eligible to be president.

The issue of constitutional eligibility, which reached the U.S. Supreme Court multiple times during Barack Obama’s runup to the White House, now is surging again, only the target this time is Cruz, who was born in Canada to an American mother and a Cuban father.

Obama states he was born in Hawaii of an American mother and a Kenyan father.

But the issue that some “birthers” raise is, what did the founders really mean and intend when they specified that a president – and only a president – is required to be a “natural born citizen.”

Many believe that, at the time the Constitution was written, “natural born” meant the offspring of two citizen parents born on the soil of the country. But the term isn’t defined in the Constitution and the U.S. Supreme Court has never ruled on that exact issue.

On his daily broadcast, Levin explained that he was “so sick of these birthers” after one man, in a hot line at a New Jersey book signing, “gets in my face, points to an obscure note … and says you were wrong about this.”

If there is a mistake in his book, “The Liberty Amendments: Restoring the American Republic,” Levin responded, he’ll fix it.

But his critic, Levin said, wasn’t finished.

“He goes, he says Ted Cruz is not eligible to be president. He’s not a natural born citizen.”

Levin explained his take on the contentious issue.

“This is not a subject I have studied so thoroughly. He was born of a mother who’s an American citizen. But he was in Canada when he was born. She wasn’t Canadian, she was an American citizen.

“So the issue isn’t what the Constitution says in that regard, the issue is how do we interpret that. His mother’s an American citizen, so he’s an American citizen. That’s not a constitutional issue. That’s an interpretation issue, or a statutory issue …. if Congress has passed some law.”

He said “birthers” get “obsessed” and that there’s “nothing I can say that’s going to change their mind.”

On Cruz? “In my mind there’s no doubt about it. He’s eligible should he choose to run.”

Hear Levin’s explanation:

An email from Tea Party Advocate, however, launched into a criticism of Levin, without explaining exactly who the man was who confronted him.

Dwight Kehoe said in the email that, “At a book signing, somewhere in the bowels of Red State New Jersey, the constitutional expert, author of books and oratorical purveyor of all things conservative (well almost all things), Mr. Levin found himself being questioned by a real American. And he did not like it. Not one bit.”

The email noted Levin’s admission he had “not really looked into it.”

“What? The guy we were all supposed to look to for law and constitutionality, the guy who has treated those that looked to him for support on this issue as pests and ‘one of those,’ has not even taken the time to ‘look into it’? That, to me, says it all. It appears he does not want to know or he is too scared to find out.”

Cruz released a copy of his birth certificate Sunday to the Dallas Morning News, to reveal his birthplace, immediately raising questions about whether he is Canadian, American or both.

He followed up just a day later by saying he would go through the motions of renouncing a Canadian citizenship.

Obama, on the other hand, is the subject of Rep. Steve Stockman’s proposed legislation calling for a congressional investigation of both the president’s constitutional eligibility and the authenticity of the birth certificate he released to show he was born in Hawaii.

In an interview with WND, Stockman said, in the case of Obama, it is more of a question about the validity of the documentation as well as his forthrightness, whereas with Cruz, it is more of a matter for legal and constitutional scholars to decide.

Stockman was happy to talk about his fellow Texan and tea-party favorite, saying, “He’s a good friend of mine and a great guy. In fact, I believe we go to the same church in Houston.”

The congressman said he doesn’t really know if Cruz is eligible for the presidency, but noted that Cruz has been upfront while Obama was not.

Stockman observed that it took a long time for Obama to produce a document, and even now, questions linger.

“One of the things I always questioned was the documentation of the president, whether that was fraudulent,” he explained. “But I don’t question Cruz. Ted came right out and said, ‘Here’s the documentation.'”

WND columnist and former adviser to British Prime Minister Margaret Thatcher Lord Monckton also has said the issue with Obama is not where he was born, but whether his documentation is authentic.

Monckton has claimed the birth certificate Obama finally produced after years of prodding is “plainly a forgery” and could be dismantled with software.

A WND book, “Where’s the Birth Certificate?” analyzed the controversy regarding Obama, going to the top of the Amazon bestseller list more than a month before it was released.

Harvard law professor Laurence Tribe, considered a leftist, and Theodore Olsen, often considered right-leaning, co-authored a legal brief for John McCain arguing he was a natural born citizen because both his parents were U.S. citizens at the time of his birth and the founders never meant to exclude from eligibility someone who was born outside the U.S., just because the parents were serving the nation in the U.S. military at the time.

WND reported previously on how the controversy over Cruz was building steam.

A News 4 report from Jacksonville noted that U.S. Rep. Ted Yoho has promised to back Texas Rep. Steve Stockman’s proposed bill to investigate the president’s birth certificate.

“So I called Steve up when I got back. He says, ‘Yeah, we’re doing it. You want to get on that?'” I says, ‘yeah,'” said Yoho.

And at ThisWeek, a report cites the questions being raised about that issue by Rep. Blake Farenthold, R-Texas.

He said Congress should have investigated Obama’s birth circumstances and eligibility, but it is probably too late to deal with the issue now.

“If we were to impeach the president tomorrow, you could probably get the votes in the House of Representatives to do it,” he said. “But it would go to the Senate and he wouldn’t be convicted.”

Several others in the House, including Reps. Jeff Duncan, R-S.C., and Markwayne Mullin, R-Okla., also recently have raised questions.

Most mainstream media outlets cite the document Obama released as proof positive, and ignore the findings of Sheriff Joe Arpaio’s law enforcement Cold Case Posse in Arizona that has stated on the record that the document is a fraud.

See the reports investigating Obama’s birth certificate.

Among the biggest names to raise concerns is billionaire Donald Trump.

Donald Trump repeatedly has insisted Obama has not documented his eligibility. At one point, he offered $5 million to the charity or charities of Obama’s choice if he would release his passport records and authorize the colleges he attended to release his applications and other records.

Trump argues that those documents would show whether or not Obama ever accepted scholarship or other aid as a foreign student.

WND has reported Mike Zullo, the lead investigator for Sheriff Arpaio’s Cold Case Posse in Arizona, said interest is rising in the Obama case.

In 2011, a contingent of citizens in Maricopa Count asked Arpaio to look into the issue because they were concerned an ineligible candidate would be on their 2012 presidential election ballot.

Zullo has concluded Obama’s birth certificate contains anomalies that are unexplainable unless the document had been fabricated piecemeal by human intervention, rather than being copied from a genuine paper document.

“Mr. Obama has in fact not offered any verifiable authoritative document of any legal significance or possessing any evidentiary value as to the origins of his purported birth narrative or location of the birth event,” Zullo said. “One of our most serious concerns is that the White House document appears to have been fabricated piecemeal on a computer, constructed by drawing together digitized data from several unknown sources.”

Zullo also has noted that the governor of Hawaii was unable to produce an original birth document for Obama, and it should have been easy to find.

See some of Zullo’s evidence:

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