"My remarks, although spoken with satirical jest, were not meant to offend or embarrass anyone -- especially the first lady," De Niro said in a statement.
The joke drew criticism Tuesday from Newt Gingrich, who said the racial reference to the Republican candidates' wives was "inexcusable" and demanded an apology from President Barack Obama.
The White House referred questions to Obama's re-election campaign. Mrs. Obama's campaign spokeswoman Olivia Alair called the joke "inappropriate" but declined further comment.
The tough-talking star of "Taxi Driver," `'Raging Bull," `'Casino," and "Meet the Parents" was host of the re-election fundraiser Monday night in New York. He opened by listing the wives of Republicans running for president.
"Callista Gingrich. Karen Santorum. Ann Romney," De Niro said. "Now do you really think our country is ready for a white first lady?"
The crowd of big-dollar donors waiting to hear from the nation's first black first lady roared in approval, and De Niro finished: "Too soon, right?"