NILES, Ohio  With the next round of voters set to weigh in on the Democratic presidential race, Senator Hillary Rodham Clinton’s campaign on Monday accused Senator Barack Obama of committing plagiarism in a weekend speech. Mr. Obama dismissed the charge as absurd and desperate.

Mr. Obama told reporters he should have credited Gov. Deval Patrick of Massachusetts, a friend, for a passage in a speech he delivered on Saturday in Milwaukee. But Mr. Obama said his rival was “carrying it too far.”

“Let’s see,” Mr. Obama said. “I’ve written two books. I wrote most of my speeches. I would add that I noticed Senator Clinton, on occasion, has used words of mine as well.”

The exchange injected a fresh dose of contention into the bitter fight for the Democratic nomination. Mr. Obama said two of his standard lines  “It’s time to turn the page” and “Fired up and ready to go”  have made their way into Mrs. Clinton’s remarks in recent weeks.