Donald Trump called "tax-hiking liberal" for past positions

Updated at 11:40 a.m. ET

Now that polls are showing that Donald Trump could be a serious contender for the Republican presidential nomination, the real estate developer and reality television personality is getting serious blowback from conservative organizations that are questioning his Republican credentials.

Trump, however, is taking it all in stride.

The influential conservative group Club for Growth on Monday released a statement calling Trump a "tax-hiking liberal whose open flirtation with single-payer health care and warm embrace of protectionism disqualifies him from consideration by conservatives."

Club for Growth President Chris Chocola called Trump's potential presidential bid a "publicity stunt [that] will sputter and disappear just as quickly as the 'The Apprentice' is losing viewers."

Trump responded to the attack on ABC's "Good Morning America," dismissing the criticisms from Club for Growth.

"Me? A liberal?" he said. "What is the Club for Growth and who are they supporting?...I had heard that they have a favored candidate, whoever that may be."

In fact, the Club for Growth is a heavy political hitter. The Center for Responsive Politics notes that Club for Growth is one of the top all-time political donors from the period between 1989 and 2010 -- even though the group was only founded in 1999. In the 2010 election cycle alone, the group spent more than $8.2 million supporting conservative candidates like Pennsylvania's new senator, Pat Toomey.

The group pointed out that in his 2000 book "The America We Deserve," Trump wrote, "We must have universal healthcare... and longer term... find an equivalent of the single-payer plan."

Single-payer health care has long been the objective of liberals who would like to see a single government entity, like Medicare, provide health care for everyone.

On ABC, Trump said of his support for universal health care, "It's a long time ago. It's 11 or 12 years ago. And frankly, a lot of people agreed with it. A lot of conservative Republicans agreed with it, at that time. We have a different country today. We can't afford things that we could have afforded or that we thought we could afford many years ago."

"My plan now is to number one get rid of Obamacare," Trump added.

In his book, Trump also called for a one-time, 14.25 percent tax on individuals and trusts with a net worth over $10 million, which the Club for Growth called "a massive tax hike on Americans that would kill jobs and investment."

Trump explained that position by again citing the changed economic climate.

"Now, we're at a point where we are so fragile and so week economically as - as a country, it's a very, very scary thing," he said. "The last thing you can be do - is - doing is raising taxes."

Trump also explained his switch from supporting abortion rights to opposing them, saying his change of heart was inspired by a friend's situation.

"He would - did not want a child and his wife didn't want a child," Trump said. "And they were going to abort. And they didn't do it for very complicated reasons. And now they have the child. And it's the apple of his eye. And he said, 'Thank God.' He changed also, by the way. 'Thank God, I didn't do it.' And I've seen that, and I've seen other things. And I am pro-life."

In an interview that aired on MSNBC this morning, Trump maintained his populist position on global trade, which has also garnered criticism from the Club for Growth.

"I would tell China very nicely...we're going to put a 25 percent tax on all your products coming in," Trump said.

He also criticized Republican Rep. Paul Ryan's proposed 2012 budget plan, which most Republicans have praised.

"I'm very concerned about doing anything that's going to tinker too much with Medicare. I protect the senior citizens," Trump said. "I think Paul Ryan is too far out front with this issue."

With many questioning whether Trump could be a viable candidate, Trump suggested that President Obama is someone who takes him seriously.

"I know many people at the White House, and one in particular, and the last person he wants to run against is Donald Trump, that I can tell you," he said.

Trump today also kept up his call for Mr. Obama to release his birth certificate (even though the president has already provided his certificate of live birth, and there is ample evidence to show the president was born in the U.S.).

On ABC, he said he would publicly release his tax returns once the president released his birth certificate.

"Maybe I'm going to do the tax returns when Obama does his birth certificate," he said. "I'd love to give my tax returns. I may tie my tax returns into Obama's birth certificate."

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