One of the biggest U.S. trade unions made an unusual dual endorsement in the 2008 White House race on Thursday, backing Democrat Hillary Clinton and Republican Mike Huckabee for their parties' presidential nominations. The International Association of Machinists and Aerospace Workers, which has about 720,000 active and retired members, backed a candidate from each party for the first time after a recent survey found about one-third of union members voted Republican and two-thirds Democratic.