Assistant U.S. Attorney Henry E. Hudson will resign his federal post today to run as a Republican-backed independent in the already combative race for Arlington prosecutor.

Hudson, 31, a former chief assistant to incumbent county prosecutor William C. Burroughs Jr., will face either his former boss or challenger John W. Purdy, who are competing to win the June Democratic primary. Hudson is expected to receive the Republican endorsement at a May 15 mass meeting.

In an interview yesterday, Hudson, who has specialized in prosecuting drug cases in his 18 months as an assistant federal prosecutor, sharply criticized the dedication and competence of Burroughs and Purdy.

"I made Bill Burroughs look good during the 3 1/2 years that I ran his office," said Hudson, a former Arlington deputy sheriff and deputy Circuit Court clerk.

Hudson called Purdy, a 48-year-old Fairfax lawyer and senior Democrat on the Arlington County Board, "one of the worst criminal lawyers I've ever seen. He has minimal criminal trial experience."

A former member of the Arlington Young Democrats, Hudson said he was seeking the Republican endorsement because "I subscribe strongly to the principles of the Republican Party. I'm a conservative person. I joined the Young Democrats in order to ensure my livelihood because my adult life has been under Democratic patronage."

Hudson said he plans to enter private law practice with Arlington attorney Claude Hilton, who will serve as a campaign adviser.