Senate District 40 is a newly-drawn district located in the heart of San Diego County. It includes University City, Mira Mesa, Rancho Bernardo, Rancho Peñasquitos, Escondido, San Marcos, Santee, Poway, Fallbrook, Lakeside, Winter Gardens, Ramona, Alpine, Valley Center, Lake San Marcos, Eucalyptus Hills, Hidden Meadows, Bonsall, Harbison Canyon, Harmony Grove, Pine Valley, Descanso, Rainbow, Pala, Del Dios, and Mount Laguna.
SD-40 is the home to nearly 1 million people, over 560,000 of whom are registered voters. 34.6% of those voters are Democrats, 34.3% are Republican, and the rest are unaffiliated or belong to smaller parties. Due to these registration figures, Politico has rated this race as one of California's most competitive.