California's 4th congressional district

• Vacaville - 102,386
• Napa - 77,480
• Davis - 66,850
• Woodland - 55,229
• West Sacramento - 53,519
• American Canyon - 21,837
• Clearlake - 15,134

• University of California, Davis - 6,805
• Hidden Valley Lake - 6,243
• Winters - 6,616
• St. Helena - 5,939
• Calistoga - 5,266
• Lakeport - 4,799
• North Lakeport - 3,514
• Angwin - 3,179
• Yountville - 2,984
• Kelseyville - 2,923
• Clearlake Riviera - 2,850
• Esparto - 2,877

In 2018, six Democratic candidates filed statements of candidacy with the Federal Election Commission (FEC).[71] They were, in alphabetical order by last name: Regina Bateson; Roza Calderon; Richard Martin; Robert Lawton; Jessica Morse; and Rochelle Wilcox. Martin and Wilcox[72] dropped out of the race, with Wilcox[73] endorsing Morse in February.

Retired Air Force Lieutenant Colonel Charlie Brown, who was the Democratic nominee for this seat in 2006 and 2008, was "seriously considering" running in 2018,[74] but decided in June 2017 against a third campaign. In January 2018, Brown endorsed Morse for the nomination.[75] Bob Derlet,[76] the Democratic nominee in 2016, also endorsed Morse in January.

On the Republican side, McClintock has one challenger, Mitchell Kendrick White, who filed with the FEC in January.[77]

In February, the California Democratic Party (CDP) endorsed[78] Jessica Morse in a contested Democratic Nomination. Roza Calderon was able to successfully collect 322 CDP-credentialed delegate signatures needed to block the endorsement, in which Morse only received 44 delegate votes. However, CDP staff refused to accept the forms after it was alleged they closed doors early to prevent the submission. A petition was later filed with the Compliance Review Commission (CRC)[79] by Calderon. The CRC voted to accept and count the signatures, ultimately disqualifying enough signatures to proceed with Morse's endorsement. California allows candidates to include their professional description under their names on the ballot. Regina Bateson later challenged Morse's ballot designation title of "National Security Fellow" at the Sacramento Superior Court after months of controversy that Morse, who had not worked in three years, was "fluffing" her credentials.[80] California's Secretary of State, Alex Padilla, had struck down Morse's 3 ballot designations before Judge Gevercer ruled[81] that she presented "no credible evidence" to use the ballot designation of "National Security Fellow". Instead, he held that this title would mislead the average person about her recent activities. In the official Certified Candidate List,[82] Morse's ballot designation was left blank.

Under the California jungle primary (aka nonpartisan blanket primary) system, only the two candidates with the most votes on June 5, regardless of party, went on to the general election on November 6. Both Republicans and four Democrats appeared on the jungle primary ballot.[78] Morse finished second in the nonpartisan blanket primary in June 2018.

Morse was denied "National Security" as her ballot designation[83] for the November ballot. In the November general election, McClintock held the district with an advantage of more than eight points.[84]