As Democrats have moved left economically, their party has also grown more concentrated in the richest parts of the country. That explains the strange spectacle of a Democratic presidential field running on the most redistributionist agenda in memory even as Democrats in Congress try to expand a tax break for high-earners in the New York City, San Francisco and Los Angeles metropolitan areas.

The GOP tax reform of 2017 capped at $10,000 the amount of state and local taxes that Americans could deduct from federal taxable income....