The following year, he moved to Denver, Colorado and practiced law. In 1902, he was declared the winner in the election as state representative, but a contest prevented his assuming his seat at the House of Representatives during the session. He began a fight for honest elections, which lasted over a decade. In 1906, he became a lawyer for the Honest Election League as well as the Law Enforcement League, the latter position he held for two years. He fought for a local option law as legal advisor, which was sustained in the Colorado Supreme Court. Costigan was chairman of the Dry Denver Committee in 1910 and was the president of the Civil Service Reform Association of Denver.