The Beltway swamp hates it, but President Donald Trump is bound and determined to shrink the federal budget and its massive deficit, and to shrink the 2 million-plus federal workforce.

Trump’s early, extraordinary steps pose a direct challenge to a fundamental underpinning of the Constitution: the power of the purse.

The Trump administration’s move to re-examine federal funding for hundreds of programs for evidence of what it calls “Marxist ...