Saving California, religiously or politically, might seem an audacious goal. Mr. Graham wants nothing less than to reform what he sees as an increasingly heathen state. But Mr. Graham and his supporters see something in California that others often miss.

Though the state has one of the highest percentages of religiously unaffiliated adults, the fast growing religious group in the country, that largely blue sea is dotted with evangelical islands that are largely red. One in five adults in the state are evangelical Christians, according to the Pew Research Center, and there are more megachurches in California than in any other state.

And while 26 percent of national voters in 2016 were white born-again Christians, according to exit polls, just 13 percent of California voters were, creating an opportunity for Mr. Graham and other conservative leaders to add to their electoral base and possibly someday swing one of the most liberal states in the country more toward purple.

Finances play a role, too. After Mr. Graham’s home state of North Carolina, California is the second largest donor base for Samaritan’s Purse and his other organization, the Billy Graham Evangelistic Association.