According to a recent Wall Street Journal poll, the four prosecutions bedeviling former President Donald Trump are deemed illegitimate by a plurality of the country. Meanwhile, the House of Representatives is heading towards an impeachment inquiry targeting President Joe Biden — which will likely be deemed equally suspect by a separate plurality of the nation. Trump’s and Biden’s legal challenges, symptoms of the country’s broader struggle with extreme partisanship, are mirror images in that each candidate’s supporters embrace the rule of law in one case while railing against its misuse in the other. It all has the feel of a...