CHAPEL HILL, N.C., March 22 _ — John Edwards, the North Carolina Democrat, said today that his wife’s cancer had returned, but that his bid for the presidency “goes on strongly.” “The campaign goes on, the campaign goes on strongly,” he said, with his wife, Elizabeth, at his side. Mr. Edwards said he learned earlier this week that the cancer had reappeared in his wife’s rib cage and that the couple recognized that it was no longer curable, though it could be managed with treatment.