A leading Montana lawyer, Walsh won election to the U.S. Senate in 1912. His 20 years' service was marked by dedication to such causes as child labour regulation, women's suffrage, the League of Nations, arms limitation, and exemption of farm organizations and labour unions from antitrust suits. By a 1922 Senate resolution, Walsh was asked to undertake a subcommittee investigation of the leasing of naval oil reserves in California and Wyoming. Through his patient, precise methods, the story of corrupt public servants' illegal involvement in the oil leases (including bribery of one Cabinet member) was revealed 18 months later. As a result, public confidence in the Harding administration was undermined. Appointed U.S. attorney general in 1933, Walsh died two days before his scheduled inauguration.