In 1955, Campbell began his career in politics when he was elected to the New Mexico House of Representatives, where he served their until 1962, where he spent his last two years serving their as a speaker. In 1962, he was the Democratic nominee for governor and defeated incumbent Edwin L. Mechem. In 1964, he became the first New Mexico governor in 12 years to win reelection. Campbell supported programs to aid the mentally ill, and appointed the first state science adviser. After leaving office, he served on the Federal Atomic Safety and Licensing Board and was president of the Federation of Rocky Mountain States, an educational foundation. He died in Santa Fe in 1999.