Lawrence O'Donnell To Steve King: 'Are You A Muslim?' (VIDEO)

On Wednesday's "Last Word," Lawrence O'Donnell and Iowa Congressman Steve King clashed over some Republicans' false belief that President Obama is a Muslim--and O'Donnell challenged King to prove that he himself is a Christian.

The interview was prompted by a focus group of Iowa Republicans, whose reaction to Bill O'Reilly's interview with Obama was aired on Fox News. O'Donnell showed footage of some of the focus group members saying they believe Obama is a Muslim.

"Do you have any doubt about whether [Obama] is a Christian?" he asked King. "I take him at his word," King said.

"Are you a Christian?" O'Donnell asked. "Yes," King said.

"Should I take you at your word or should I maybe suspect you are Muslim?" O'Donnell said. "Do you have a Christian ID you can show me?"

King said that the misconception about Obama's religion among many Republicans is "not a Republican problem, this is the president's problem." O'Donnell asked,"if Republicans believe the world is flat, is that a Republican problem or a geography problem?"