Property tycoon turned White House opponent Donald Trump on Tuesday again claimed there was a doubt about Barack Obama's legality as president.
"There is a real question about his own citizenship," the colorful and polarizing businessman told ABC television.
The comments came just a day after influential Republican strategist and former George Bush advisor Karl Rove said he was a 'joke candidate'
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
In response to questions about why Trump will not make public his tax details, Trump said: "Maybe I'm going to do the tax returns when Obama does his birth certificate. I'd love to give my tax returns. I may tie my tax returns into Obama birth's certificate."
Trump, a household name thanks to decades of sometimes controversial building projects and a TV reality show, is running an unofficial White House campaign ahead of 2012 elections and says he will announce a formal decision soon.
His most attention-grabbing gambit so far has been to join rightwing conspiracy theorists who claim Obama was not born on US territory, which would disqualify him for the presidency. Official records show Obama was born in the state of Hawaii.
However "The Donald" has little chance of besting Obama if an election were held today, the latest ABC/Washington Post poll finds.
The poll said that if Trump ran as Republican he would win 34 percent of the vote against 49 percent for Obama.
Trump has not indicated whether he would run as Republican or a deep-pocketed independent.
The poll also found that voters have little enthusiasm for current Republican frontrunners, with only 43 percent satisfied. In a primary today, Mitt Romney would get 16 percent, Trump would get eight and Mike Huckabee, six.
And parts of the GOP (Grand Old (Republican) Party) are not happy he is running.
"His full embrace of the birther issue means that he's off there in the nutty right and is now an inconsequential candidate,", the Huffington Post quoted Rove as saying.
"I'm shocked, the guy is smarter than this."
Left-leaning Washington Post columnist Eugene Robinson said Trump had to be taken seriously.
"Questioning the president's birthplace obviously began as a ploy to grab attention, and it worked, but then swelled into a central theme of Trump's candidacy as he gained traction among the conspiracy theorists who actually believe such nonsense."
