Silicon Valley voters this November will cast ballots in one of the region’s most consequential elections: From deciding who to send to Washington to a historic county supervisorial election, there is no shortage of major decisions up and down the ballot.

Several prominent politicians are lining up to replace termed-out state Sen. Jim Beall in an election that’s more than a year away. San Jose Councilman Johnny Khamis, a former Republican who fled the party in June over Donald Trump’s immigration policies, confirmed that he’ll run for the seat. Khamis discussed the idea with political consultant...