Chancellor Angela Merkel’s heir apparent pulled out of an election rally on Thursday amid a plagiarism scandal that could cost him credibility, his Ph.D. title, and possibly even his job. On Wednesday, a newspaper reported that Defense Minister Karl-Theodor zu Guttenberg had lifted passages of his law dissertation without correctly attributing them in footnotes or a bibliography. On Thursday, more media outlets came forward with further parts of the thesis that they said were of questionable origin. Mr. Guttenberg has dismissed the accusation that he passed off others’ work as his own, but made no comments on Thursday.