"And if the challenger won over Chaffee in Rhode Island primary would he lose in the Fall election?"
Rhode Island will be reelecting conservative Republican Governor Carcieri (who won with 55% in 2002) this November, so Mayor Laffey would have a real chance against Attorney General Whitehouse (yes, that's the Democrat candidate's name). And if Chafee runs as an independent instead (as has been speculated of late), it will be a humdinger of a 3-man race with the winner getting less than 40%, which will give Laffey an excellent chance of victory.