Donald Trump will “probably” run as an independent candidate for U.S. President in 2012 if he does not receive the Republican party’s nomination, he told the Wall Street Journal in a video interview on Monday.

Mr. Trump’s candidacy would complicate matters for the GOP as it looks to front someone who can unite the fractious party and mount a serious challenge to President Obama’s reelection bid. A recent Wall Street Journal/NBC poll recently found Mr. Trump tied for second place with former Arkansas governor Mike Huckabee among likely voters in a GOP primary. Mitt Romney, the former Massachusetts governor who moved a step closer to formally declaring his own candidacy Monday, is still the frontrunner, though not by a wide margin.

“I think the Republicans are very concerned that I [may] run as an independent,” Mr. Trump said. His support is highest among the conservative wing of the party, not least because he is among the so-called “birthers” who doubt that President Obama in fact was born in the U.S. “It’s a very important issue,” Mr. Trump said of demanding that President Obama show his birth certificate, which has separately been reviewed by the media and deemed legitimate. “I’m not ashamed of having raised that issue.”