Nearly half of Iowans supporting real estate mogul Donald Trump’s presidential campaign don’t believe President Barack Obama was born in the United States, according to a poll released Tuesday.

The Bloomberg Politics/Des Moines Register poll found that 35 percent of likely Iowa Republican caucusgoers don’t believe the President was born in the U.S. That “birther” share rose to 46 percent among Trump supporters, the poll found.

Trump took credit for Obama releasing his long-form birth certificate back in 2011. The billionaire claimed at the time to have a team of investigators on the ground in Hawaii looking for proof that the President was indeed born in Honolulu.

The poll surveyed 400 likely Iowa Republican caucusgoers by phone from Aug. 23-26. It had a margin of error of plus or minus 4.9 percentage points. The pollsters noted subsets like Trump supporters had a larger margin of error but did not provide the breakdown.