Even after the release of his long-form birth certificate last week, a substantial amount of Republicans still do not believe President Barack Obama has proven he was born in the United States.

A new Zogby interactive poll conducted April 29 to May 2 of 2,020 likely voters found that 30 percent of Republicans and 21 percent of those without a college degree did not believe Obama was born in the U.S. Sixteen percent of all voters said they did not believe Obama has proven he was born in America.

Many so-called "birthers" believe there is persuasive evidence that Obama was born in Kenya in 1961 and that his birth certificate was faked in order to make him eligible for the Presidency. The conspiracy was first conceived during the 2008 campaign.

Reality TV star Donald Trump brought the issue back into the media spotlight when he first revealed he had some doubts that Obama was a U.S. citizen during an interview with ABC News' Ashleigh Banfield in March. Trump had been using the issue to court the media's attention.

On April 27, Obama released his long-form birth certificate, which confirmed that the president was born in Honolulu, Hawaii on August 4, 1961.