Cox previously ran for the United States House of Representatives in in the 2006 elections, losing to incumbent George Radanovich. In the 2018 elections, Cox again ran for the United States House of Representatives, this time in. Cox began this congressional bid in 2017, competing in California's 10th district primary race against several other Democratic candidates. However, Emilio Huerta, the only Democratic challenger in the 21st district, withdrew from the race prior to the filing deadline to appear on the primary election ballot. Cox withdrew from the 10th district race to instead run in the 21st district against incumbent Representative David Valadao. He and Valadao advanced from the June 5 top-two primary election to the November 6general election. On election night, and for several days after the election, Valadao had more votes, but Cox's vote count pulled into the lead on November 26. By November 28, major news sources called the race for Cox, with Valadao conceding the race the following week. Cox's victory was considered an upset, as most election forecasters rated Valadao as the favorite. Cox won by a narrow 862 vote margin.
2020
Cox is running for reelection in 2020, and Valadao will challenge Cox in an attempt to reclaim his former seat. Roque "Rocky" and Ricardo De La Fuente have also filed to run, with Rocky entering the race as a Republican and Ricardo as a Democrat.
When Cox made an updated financial disclosure in 2019, it was discovered that he had failed to disclose business interests as a candidate in 2018. It was also discovered that Cox failed to timely pay wages owed to three employees of Constellation Mines, a company where Cox was a director until early 2019. These obligations were not disclosed when Cox ran for Congress in 2018. In March 2020, Cox voted against a bill that would require members of Congress to disclose tax liens. In January 2020, the IRS placed a tax lien on Cox for approximately $87,000 in unpaid income tax for 2016 and approximately $57,000 in unpaid income tax for 2017. Cox was also subject to a $50,000 IRS tax lien in 2017, which Cox claims was the result of his check being stuck to the back of someone else's payment.
Electoral history
Personal life
Cox has four children with his wife, pediatrician Kathleen Murphy.