The California top-two system started in 2011. Ever since, no third party candidate for Congress has been on the ballot in November, except in 2014 a Peace & Freedom Party member, Adam Shebeita, qualified in the 44th district. He achieved that because only one person filed to be on the primary ballot in that district and Shebeita placed second as a write-in in the primary.

But in 2018, there are two U.S. House races in which minor party candidates will appear on the November ballot, because the only person who filed, other than the minor party candidates, was the Democratic incumbent. One of these districts is the 40th, in which the only two candidates on the primary ballot are the incumbent Democrat and a Green, Rodolfo Cortes Barragan. Thus Cortes Barragan will be the first Green to be listed in November for U.S. House, since California started using top-two.

The other district is the 34th, in which three candidates filed to be on the June ballot, the Democratic incumbent, a Green, and a Libertarian.

Cortes Barragan has an article about these two races at the Independent Voters Network.

There are also two races in which only the incumbent Democrat filed to be on the primary ballot, so it is possible a minor party write-in candidate might place second. Those are the 13th and 32nd districts.

Rex Rammell, a Wyoming veterinarian who has run for office several times as a Republican, and who had been seeking the Republican nomination for Governor, announced on May 10 that he would instead join the Constitution Party and seek its nomination for Governor. Here is his campaign web page, which doesn’t yet mention his party switch.

Here is a newspaper story about his party switch.

On May 10, the California Secretary of State released a new voter registration tally, as of April 6. This is the first California tally since the tally of January 2, 2018.

The only parties that increased their share of the registration since the January 2018 tally are the American Independent and Libertarian Parties. The number of all registered voters declined slightly since the January 2018 tally, owing to purges of the rolls.

The new percentages are: Democratic 44.58%; Republican 25.27%; American Independent 2.664%; Libertarian .7405%; Green .4798%; Peace & Freedom .39%; unknown .15%; unqualified parties .608%; independent 25.11%.

The January 2018 percentages were: Democratic 44.63%; Republican 25.44%; American Independent 2.655%; Libertarian .738%; Green .483%; Peace & Freedom .40%; unknown .11%; unqualified parties .607%; independent 24.95%.

The Constitution Party, which is not ballot-qualified, declined from 321 members to 312 members.