The Democratic competition to bleed the rich has become fierce. If progressives aren’t satisfied with Elizabeth Warren’s proposed “wealth tax” of 3% a year, AOC’s 70% income tax, Bernie Sanders’s 77% estate tax, or Rep. John Larson’s uncapped payroll-tax increase of 2.4 percentage points, they now have a fifth option. Oregon Senator Ron Wyden wants to tax capital gains as regular income, meaning rates up to 37%. He would also tax unrealized gains, perhaps decades before the investor sells.

Mr. Wyden hasn’t released a formal...