King suggested that Obama's middle name, which is Hussein, would have a special meaning for "radical Islamists".
"His middle name does matter," he said. "It matters because they read a meaning into that."
Obama is mixed-race and a Christian who says he has little connection to Islam.
"These comments have no place in our politics," Tommy Vietor, a spokesman for Obama, said.
He also called on John McCain, the Republican nominee, to "repudiate them like he has previous offensive comments from his supporters".
Last month, McCain denounced an introduction by Bill Cunningham, a Cincinnati talkshow host, who referred to Obama three times as "Barack Hussein Obama".
King said his comments were not meant to demean Obama but to warn how an Obama presidency would look to the world.
Obama has said that he favours withdrawing US troops from Iraq.
In 23 caucuses around Wyoming on Saturday, Democrats, who make up about 25 per cent of the staunchly Republican state's electorate, chose Obama by 61 per cent to 38 per cent.
The outcome means Obama will win most of the 12 delegates at stake, a tiny number compared to the 2,025 needed to secure the Democratic party's presidential nomination at its August convention.
After Wyoming, Obama and Clinton will face voters in the southern state of Mississippi, where 33 delegates are at stake.
The campaign will then move to Pennsylvania on April 22.