A Republican primary challenge to Senator Chafee of Rhode Island is attracting backing from national groups that favor tax cuts and a strong U.S.-Israel relationship. The mayor of Cranston, R.I., Stephen Laffey, 43, is hoping to unseat Lincoln Chafee, a Republican who was appointed to the Senate in 1999 after the unexpected death of his father, John Chafee, and who won election to his father's former seat the following year.