LOS ANGELES — Once the 2020 presidential campaign leaves the early political tests in Iowa, New Hampshire, Nevada and South Carolina, the political themes will be California themes, the issues will be California issues and the values being debated will be California values.

And that will be the case not only because Californians will be able to engage in early voting for their own March 3 primary on the morning of Feb. 3, hours before Iowans trudge in cold and snow to precinct caucuses in church basements, town libraries, middle-school auditoriums and fraternal halls to begin the process of selecting delegates for the Democrats’ nominating convention in Milwaukee next summer.

For the first time in modern American history, it is one state, and that state’s preoccupations, that are the major engines of a vital presidential election. And that was on broad display in June, when all the major candidates except former Vice President Joseph R. Biden Jr. strutted their stuff at the California Democratic Convention and a MoveOn.org conclave in San Francisco.

“California issues and California sensibilities are at the heart of this election,” said Mindy Romero, the Sacramento-based director of the USC California Engagement Project. “California may be different from the rest of the nation, but our issues are the crux of the national debate. And we have a president who has fanned the idea that California values are counter to the values of the rest of the nation.”

This election — occurring as the threat of the decline of the American dream collides with the persistence of California Dreaming — is a test of that notion.

Like states big and small, California and its electorate are not monolithic. California, after all, spawned both the conservatism of William F. Knowland and the liberalism of the Berkeley Free Speech Movement, and was the birthplace of both Haight-Ashbury flower power and Breitbart News alt-right disruption.

But overall California, which has voted Democratic the past eight elections, today has a discernible left-leaning political profile. Its domination of the political debate could only occur in an election cycle that does not have a “the-economy-stupid” theme but where issues such as health care, immigration and trade are at center stage.

Eighty years ago, in the year of the Golden Gate International Exposition celebrating the completion of the Golden Gate and Bay Bridges, the Depression-era Federal Writers Project guide to California spoke of the state’s “metropolis of -isms.” Less than a decade later, in 1946, the revered American journalist and author John Gunther opened his best-selling “Inside U.S.A.” with a survey of California, describing it as “the most spectacular and most diversified American state,” and going on to speak of a “California so ripe, golden, yeasty, churning in flux … a world of its own.”

Today the state remains golden, yeasty and churning in flux — and -isms persist. But now, the result of relentless population growth and the peculiarities of the Electoral College, the state’s electoral power is greater than the critical swing states of Wisconsin, Ohio, Pennsylvania and New Hampshire — the four likely principal battlegrounds of the 2020 campaign, and where the presidency will be determined — combined.

And so the evolution of California as the dominant factor in the country’s politics is almost a manifest destiny deferred, tardy in its arrival even though the state in the mid-19th century produced the first Republican presidential nominee and then in the 20th century accounted for three Republican presidents.

The state’s future orientation is best seen in its efforts to battle climate change.

“The environment,” said Pat Smith, executive director of EarthShare California, a coalition of top environmental groups in the state, “is one of our fundamental values.” So much so that the state government has filed some two dozen environmental-related lawsuits against the Trump administration and has prevailed in more than half of them.

The Trump tariffs also have emerged as a major issue in this state, a major agricultural exporter.

“Our industry has always wanted open markets, and we want to see the current situation corrected,” said Richard Waycott, president of the Almond Board of California, which represents the interests of the state’s 7,300 almond growers, virtually all family farmers, and the 100 processing companies that trade, package and ship the nuts. Altogether the almond industry supplies more than 80% of the world’s supply and accounts for about $4.2 billion in trade.

Immigration, too, is an important issue here.

“The economic fabric of California always has been based on the labor of migrants and is especially so now,” said Victor Narro, who directs the UCLA Labor Center and is a professor in the Labor and Workplace Studies Program at the university.

While there is enormous overlap between what is central to California voters and what will be the principal issues in the 2020 campaign, an important part of the enhanced influence of California is the bigger role that the state will play in selecting Donald Trump’s opponent. Next year’s California primary will occur two months earlier than in 1984, when Sen. Gary Hart of Colorado won the June 5 contest, but former Vice President Walter Mondale picked up sufficient delegates to claim the Democratic presidential nomination.

“If we were back in the pack, back in June again, the power of California to dominate the presidential campaign might not be the case,” said Mark DiCamillo, director of the Berkeley Institute of Government Studies Poll.’’

This enormous Democratic candidate field, moreover, is another congruence with California. “That is reflected in the kinds of candidates — a group that is very diverse and in that respect it’s what California is really about,” said Ann Crigler, director of USC’s Jesse M. Unruh Institute of Politics.

That is perhaps why an insight now a half-century old has fresh relevance. “California,” Joan Didion once wrote, “is a place in which a boom mentality and sense of Chekhovian loss meet in uneasy suspension; in which the mind is troubled by some buried but ineradicable suspicion that things had better work out here, because here, beneath that immense, bleached sky, is where we run out of continent.”

David M. Shribman is a nationally syndicated columnist.