A full-throttle backlash against Donald Trump took hold in the United States on Thursday, with everyone from civil rights activists to respected journalists and a Tea Party favoured son piling on.

Even the usually unflappable Trump bristled at accusations he was racist for raising questions about Barack Obama's academic credentials on the heels of suggesting the president wasn't born in the United States.

Accustomed to being the subject of widespread ridicule in the 1980s, when Canadian Graydon Carter's Spy magazine gleefully skewered him as a "short-fingered vulgarian," Trump had since established a broad fan base and a grudging respect for his business acumen due to his reality show, The Apprentice.

But there was little love for Trump on Thursday, the day after the White House released the president's long-form birth certificate after the billionaire rejuvenated the so-called birther debate. The standard form of the document was released three years ago.

Trump took credit for the White House move, then proceeded to his next line of attack by suggesting Obama was an academic fraud who didn't have the grades to get into Harvard. It seemed a reference to the country's affirmative action policies, loathed by many Republicans.

Racism alleged

That new battle front prompted a scathing rebuke from Bob Schieffer, a respected journalistic veteran in the United States who's covered national politics for decades.

"That's just code for saying he got into law school because he's black," Schieffer said on the CBS Evening News. "This is an ugly strain of racism that's running through this whole thing."

Jesse Jackson and other civil rights activists have also suggested Trump's stances on both the birther issue and Obama's academic bona fides smack of racism. A white president, they point out, has never been hounded by questions about his birthplace.

A day after Schieffer's remarks, Trump seemed taken aback.

"That is a terrible statement for a newscaster to make," Trump said when reached by the gossip website TMZ. "I am the last person that such a thing should be said about."

But for the first time since Trump began ostensibly testing the waters for a run for the Republican presidential nomination, it appeared he blinked. The man known as "the Donald" walked back his latest Obama attack.

"Grades are the least important aspect of somebody being president," he told TMZ. "It's not a big subject for me."

The backlash was brewing even before a frustrated Obama, decrying the "carnival barkers" who erroneously insist he wasn't born in the U.S., appeared in the White House briefing room to express dismay that questions about his birthplace continued to dog him.

Ratings decline

Liberal fans of Celebrity Apprentice have reportedly stopped watching since Trump started flying the birther flag and consequently the show's ratings are dropping, the Atlantic reported Thursday, citing demographic research provided by National Media Inc.

Conservative pundits like Karl Rove and Charles Krauthammer have been contemptuous of Trump's potential run for president for weeks. And privately, Republican insiders have been appalled at how much attention he's been diverting from other, more legitimate, potential candidates who might stand a chance of beating Obama in 2012.

Trump has been atop polls, leading other potential Republican candidates.

But Rand Paul, a darling of the Tea Party movement — whose adherents largely believe Obama wasn't born in the United States — said Trump's numbers will plummet when Republicans learn that he's donated more money to Democrats than the GOP, including to Harry Reid, the Democratic Senate majority leader.

Paul issued a challenge to Trump on Thursday in public comments that are among the first to spring publicly from the lips of a high-profile Republican.

The Kentucky senator opened a breakfast speech in New Hamsphire to demand Trump's Republican credentials.

"I want to see the original long-form certificate of Donald Trump's Republican registration," he said to laughter from the crowd. "Seriously ... I want to see the original long-form certificate, with embossed seal, of Donald Trump's Republican registration."

He even took a jab at Trump's attacks on Obama while ridiculing his simplistic ideas on how to deal with soaring gasoline prices.

"He's always complaining about the president's education," Paul told reporters covering the event in Concord, N.H.

"What economic school teaches you that you can have a bully for a president who sets the prices by just telling countries what price they should charge? That to me shows an economic simplicity that really may not be equivalent to the stature of being president."