Surrogates for Mr. Trump mused aloud on Tuesday about what might have happened, raising questions about fissures within his team and allowing the controversy to drag on.

One adviser to Mr. Trump, who has assisted in the drafting of some of his speeches, acknowledged that Ms. Trump used words that were not her own. “I’m sure what happened is the person who was helping write this plucked something in there and probably an unfortunate oversight — and certainly Melania didn’t have anything to do with it,” the adviser, Sam Clovis, a Trump campaign co-chairman, said in an interview on MSNBC.

Katrina Pierson, another spokeswoman for the Trump campaign, acknowledged that Ms. Trump used phrases similar to those used by Mrs. Obama but insisted that the language was not copied verbatim.

She said in an interview with Sky News that Ms. Trump was trying to echo themes expressed publicly by prominent women including Laura Bush and Elizabeth Dole.

“She really wanted to communicate to Americans in phrases they’ve heard before,” Ms. Pierson said.

Talk of who was to blame for the speech also buzzed among former advisers to Mr. Trump. Corey Lewandowski, the campaign manager who was ousted in favor of Mr. Manafort, said that Mr. Manafort should take responsibility.

“Whoever signed off, the final signoff that allowed this to go forward, should be held accountable,” Mr. Lewandowski said on CNN. “I think if it was Paul Manafort, he would do the right thing and resign.”