LOS ANGELES—Accusations that a campaign treasurer stole more than $1 million from Democratic candidates across California have jolted the party on the eve of the 2012 campaign season. Kinde Durkee was arrested Sept. 2 and accused by federal authorities of stealing campaign funds and using the money to pay for an array of personal expenses, including mortgage payments, cosmetics and nursing-home care for her mother. Based in Burbank, Calif., the 58-year-old had long managed money for scores of Democratic campaigns. According to the federal criminal complaint, Ms. Durkee "misappropriated money from her clients' bank accounts and filed false disclosure reports...