The feud between Elon Musk and Donald Trump is a godsend for Democrats, a headache for the president and a problem for Mike Johnson, the Republican speaker of the House. Between now and the Fourth of July, Trump expects congressional Republicans to deliver a badly needed win. After more than four months back in the Oval Office, he still lacks a major legislative achievement. Republicans control both the House and Senate, but the public sees goose eggs on the scoreboard. Almost daily, the courts upend the president’s executive orders. Slim legislative majorities and government by rage come with drawbacks. Trump’s...