Miller, who is retiring from the Senate next year, has often expressed his admiration for Bush. He was a co-sponsor of the president's tax cuts in 2001 and 2003. The two got to know each other in the 1990s when both were governors.

The senator's endorsement is important for several reasons. With Miller on board, Bush will have a head start on forming a Democrats for Bush group in 2004. Such a group would woo crossover votes from conservative or otherwise disgruntled Democrats next year. In 2000, an effort by the Bush campaign to form a Democrats for Bush organization fizzled.

Since he came to the Senate in 2000, Miller has become increasingly critical of Senate Democrats and the national Democratic party. He recently published a new book, "A National Party No More: The Conscience of a Conservative Democrat," in which he criticizes the party for being too liberal, too elitist, and subservient to liberal interest groups. In the book, Miller singles out Democratic presidential frontrunner Howard Dean, whom he knew as governor of Vermont, for being shallow.