Donald Trump, the celebrity business tycoon once dismissed as a "carnival barker" by U.S. President Barack Obama, went on a full-fledged, multi-media tear on Tuesday, doubling down on his insistence that America's first black president was born in Kenya.

On the day he appeared alongside Mitt Romney at a Las Vegas fundraiser for the Republican nominee, Trump belligerently defended himself against accusations he had lost his grip and was hurting the man who hopes to replace Obama in the Oval Office in November.

His behaviour, in fact, overshadowed what was meant to be a triumphant day for Romney — the former Massachusetts governor finally sealed the Republican presidential nomination by winning the Texas primary on Tuesday night, and received a congratulatory call from Obama on Wednesday.

"I walk down the street and people are screaming: 'Please don't give that up,"' Trump told CNBC in response to a new Obama ad that maligns Romney for failing to distance himself from the real estate mogul's resurrected conspiracy theories about the president's birthplace.

When Trump flirted with his own run for the Republican nomination last year, he put the so-called birther issue in the national spotlight. At one point, he claimed he had hired "investigators" to probe the authenticity of Obama's Hawaiian birth certificate.

Soon after, the president released the long-form version of the document that proved once again he'd been born in Honolulu on Aug. 4, 1961. Visibly irritated, he decried Trump as a "carnival barker" during a news conference in April 2011 at the White House.

But on Tuesday on CNBC, Trump insisted there still are large numbers of Americans who share his suspicions. Later in the day, he engaged in a verbal brawl with CNN's Wolf Blitzer, who told him he was sounding "a little ridiculous."

"You're beginning to sound a little ridiculous," Trump responded, making repeated snide remarks about CNN's falling ratings. "There are many people who do not believe that birth certificate is authentic."

When asked to provide the name of a single person in authority who believed the document was fake, he replied: "I don't give names." He also wouldn't respond when asked what his investigators had uncovered in Hawaii last year.

The drama played out just hours before Trump hosted the Romney fundraiser in Las Vegas.

The Romney campaign responded to the fresh Trump accusations by reminding the media that the candidate has repeatedly said he believes Obama was born in the U.S.

But on the eve of the Las Vegas fundraiser, Romney didn't go quite that far.

"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 Monday when asked if he was comfortable appearing with Trump as he renews his birther accusations.

Trump's fit of pique was apparently sparked by the new Obama ad, set to circus music, that assails Romney's "refusal" to condemn him. Not only were the two men appearing together at the fundraiser, but Romney's campaign was even raffling a chance to have dinner with star of The Apprentice for anyone who donated $3.

Revisits skepticism

The day had barely dawned when Trump took to Twitter to attack the ad, accusing Obama of "practically begging" Romney to "disavow the place of birth movement, he is afraid of it and for good reason."

Trump revisited his skepticism about Obama's birthplace last week in an interview with The Daily Beast.

He pointed to erroneous promotional material for Obama's 1991 memoirs that falsely described him as Kenyan-born. The memoirs were later published in 1995 as Dreams From My Father.

"A book publisher came out three days ago and said that in his written synopsis of his book," Trump said. "He said he was born in Kenya and raised in Indonesia. His mother never spent a day in the hospital."

A former employee for the publisher recently said the information about Obama being born in Kenya was their mistake.

"Now they're saying it was a mistake," Trump said. "Just like his Kenyan grandmother said he was born in Kenya, and she pointed down the road to the hospital, and after people started screaming at her she said: 'Oh, I mean Hawaii.' Give me a break."

'Bloviating ignoramus'

His comments caused one high-profile conservative pundit, George Will, to label Trump a "bloviating ignoramus" with a low IQ. Will also warned in a weekend TV appearance that Romney's association with Trump would hurt his campaign.

Trump responded in kind on Twitter.

"George Will may be the dumbest (and most overrated) political commentator of all time. If the Republicans listen to him, they will lose," he wrote.

On Tuesday, he spewed further invective Will's way, calling him a "loser" who used him to get publicity for himself.

In the Obama video, entitled "Two Republican Nominees," Romney is contrasted unfavourably with one of his biggest congressional supporters, Arizona Senator John McCain.

When McCain ran against Obama in 2008, the ad begins, he "stood up to the voices of extremism in his party."

McCain is then seen chiding a woman who insists, during a town hall, that Obama is "an Arab." The senator shakes his head and responds forcefully: "No ma'am, no ma'am. He's a decent family man … that I just happen to have disagreements with."

Mocks Obama campaign ad

"Why won't Mitt Romney do the same?" asks the video before a montage of clips showing Trump questioning Obama's birthplace.

Trump mocked the ad's premise that Romney should take a page from McCain's playbook.

"(Obama) keeps using @SenJohnMcCain as an example, however, @SenJohnMcCain lost the election. Don't let it happen again," he tweeted.

Trump announced he wasn't going to run for president last May, about a month after Obama released his long-form birth certificate. The perma-tanned businessman with the trademark orange-hued hairstyle was said to have been personally wounded by the ridicule directed his way as he tested the waters for an official campaign.

Indeed, at the White House correspondents dinner a few days after Obama released the document, Trump sat glowering petulantly as the president made repeated jokes at his expense that were greeted with roars of laughter and applause from the crowd.