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WASHINGTON — President Trump has never been one to dive into the details of policy, especially now, as an impeachment inquiry threatens to engulf his administration. In recent days, he has allowed his son-in-law to lead a major policy shift in the Middle East and has backed away from a high-profile promise to ban flavored e-cigarettes.

But even as his presidency teeters, one of the few policy issues that has maintained Mr. Trump’s personal focus is not one central to his political appeal, like immigration or trade.

It is the state of California.

The state has been a political fixation since the early days of his presidency, but that was heightened this autumn. Mr. Trump has attended meetings, asked detailed questions at briefings and pressed aides to find ways to use policies to go after the most populous state in the union, according to three people familiar with the matter. Aides say that Mr. Trump remains deeply involved on immigration policy, like a recent decision to slash the nation’s refugee program nearly in half, and on trade. But they describe him as obsessed with narrow policies that directly affect California. Beyond those three policy matters, little else has penetrated the swirl of impeachment.

Two Californians, Speaker Nancy Pelosi and Adam B. Schiff, chairman of the House Intelligence Committee, are leading the impeachment investigation. The state’s Democratic governor, Gavin Newsom, has been unsparing in his criticism and his legal challenges.