(CNN) -- Donald Trump did not back down Tuesday from his questioning of President Barack Obama's birthplace, instead arguing heatedly with CNN's Wolf Blitzer that "a lot of people do not think it was an authentic certificate," though he declined to offer names of experts who agree with him.

The issue of Obama's birth -- litigated in the 2008 presidential campaign, then settled when the Obama White House released the long-form version of the certificate in April 2011 -- is being brought to the fore in no small part by Trump, who is also set to fundraise alongside presumptive Republican presidential nominee Mitt Romney Tuesday evening.

On CNN's "The Situation Room" Tuesday, Trump maintained that Obama's birthplace is a matter of opinion, rather than fact.

"Everybody's entitled to your opinion," he said. "You know my opinion and you know his opinion and that's fine. We're entitled -- as he said yesterday in the airplane -- we're all entitled to our opinions and he's entitled to have his opinion. I don't happen to share that opinion, it's wonderful."

Blitzer presented Trump with newspaper announcements of Obama's birth from 1961.

Trump interrupted, "Can you stop defending Obama?"

"Donald, you're beginning to sound a little ridiculous, I have to tell you," Blitzer replied.

"You are, Wolf," Trump fired back. "Let me tell you something, I think you sound ridiculous."

Trump then alleged that the practice of filing U.S. birth announcements for an overseas birth was commonplace, though he offered no evidence.

"Many people put those announcements in because they wanted to get the benefits of being so-called born in this country," Trump said. "Many people did it."

Obama's long-form birth certificate showed he was born in Hawaii on August 4, 1961 and was acknowledged as authentic by state officials. Natural born American citizenship is one of the Constitutional requirements for the presidency.

"Is it the most important thing?" Trump asked in an interview earlier on Tuesday. "In a way it is. You're not allowed to be the president if you're not born in the country."

Trump finds himself at odds with GOP presidential candidate Mitt Romney, whose spokeswoman pointed out in an email statement that "Governor Romney has said repeatedly that he believes President Obama was born in the United States."

But Romney made no effort to distance himself from the comments, even when presented with the opportunity by reporters on Monday.

"You know, I don't agree with all the people who support me and my guess is they don't all agree with everything I believe in," Romney said when asked about the issue. "But I need to get 50.1% or more and I'm appreciative to have the help of a lot of good people."

Trump said he has not spoken about the birther issue with Romney

"I haven't talked to Mitt Romney about it," he said. "What I speak to Mitt Romney about is jobs. What I speak to Mitt Romney about is China, because he's got a great view on China and how they're taking our jobs and taking our product and manipulating their currency so it makes it almost impossible for our companies to compete."

Trump spoke by phone with Blitzer in the interview from Las Vegas, where he and Romney will fundraise with former GOP presidential candidate Newt Gingrich.

Obama's campaign released a web video on Tuesday that said the 2008 GOP nominee "John McCain stood up to the voices of extremism in his party... Why won't Mitt Romney do the same?" In the video, McCain is seen addressing a woman who called Obama an "Arab," saying, "He's a decent family man, citizen that I just happen to have disagreements with."

The president's deputy campaign manager suggested Romney's alliance with Trump in light of the tycoon's statements calls into question how he would serve as president.

"Mitt Romney's continued embrace of Donald Trump and refusal to condemn his disgraceful conspiracy theories demonstrates his complete lack of moral leadership," Stephanie Cutter said in a Tuesday afternoon statement. "If Mitt Romney lacks the backbone to stand up to a charlatan like Donald Trump because he's so concerned about lining his campaign's pockets, what does that say about the kind of President he would be?"

White House Press Secretary Jay Carney said Tuesday the White House was forced to address the issue last year, "but not because we chose to, but because it was such a ridiculous distraction from the important business that we should be doing here that the president is committed to doing and Congress should be doing to help the economy grow and help it create jobs."

"We can revisit that but I think the American people are pretty fed up with this kind of nonsense," he said.

As evidence of his claims, Trump pointed to a biographical note published in the 1990s by Obama's literary agent, which said Obama was born in Kenya.

The literary agent recently apologized for the mistake.