ON Feb. 9, 1950, Senator Joseph McCarthy claimed his place in the history books by telling a crowd in Wheeling, W.Va., that the State Department was full of Communists. “We are not dealing with spies who get 30 pieces of silver to steal the blueprint of a new weapon,” he said. “We are dealing with a far more sinister type of activity because it permits the enemy to guide and shape our policy.” The claim was baseless, scurrilous  and plagiarized. The same words, practically verbatim, had been spoken on the floor of the House of Representatives two weeks earlier, by Representative Richard M. Nixon of California.

Senator McCarthy’s wholesale borrowing was discovered only years later. Had reporters noticed it sooner, he might have run into a different kind of trouble than he did, for the press loves a plagiarism quarrel. Consider the well-aired sins of numerous writers in recent years  and last week’s back-and-forth over Senator Barack Obama’s uncredited use of the words of others.

Last weekend it was reported that Mr. Obama used on the presidential campaign trail a rhetorical set-piece first spoken by Gov. Deval Patrick of Massachusetts, a friend and co-chairman of his campaign. The sequence contained famous political lines followed by the refrain “Just words”  a gibe meant to rebut the taunt of his rival, Senator Hillary Rodham Clinton, that he offers voters only speeches, not deeds. Mr. Obama acknowledged that the failure to cite Mr. Patrick was an error, if an unimportant one. In Thursday’s debate, Mr. Obama said he thought it was “silly” that this was even under discussion. Mrs. Clinton pressed the case, saying, “If your candidacy is going to be about words, then they should be your own words.”

Mr. Obama is unusual among politicians for having written a memoir praised for its literary skill and for being the author of at least some of his own finely wrought speeches. That reputation is partly why the suggestion of plagiarism was startling to some. Hendrik Hertzberg, who was a speechwriter for Jimmy Carter, wrote on his blog at newyorker.com that Mr. Obama’s was “not a mortal sin,” but rather a “damaging mistake ... given that Obama’s eloquence and ‘authenticity’ are so central to his appeal.”