The Democrat has endorsed candidate Rudy Salas for the 2022 congressional race.
The District 21 seat has flipped in the last two elections.
In 2018, Cox defeated Republican incumbent David Valadao.
Last year, Valadao beat Cox to regain the office.
State Assemblyman Rudy Salas announced his run for Congress last month.
The Bakersfield native is one of almost a dozen candidates looking to challenge Valadao.