The administration’s attacks are a case study in how executive power can be abused.For a month, President Donald Trump has been picking off his enemies among America’s top law firms. They have been vulnerable in part because they didn’t stand together. But some leading firms are now fighting back aggressively in what could be one of the epic battles of Trump’s presidency.Trump’s campaign against the major law firms has tested the guardrails of the Constitution as much as any other action since Inauguration Day. Trump has declared his motive: He wants revenge against lawyers who investigated him during and after...