MOUNT RUSHMORE, July 4 -- Rain and hail swept across Keystone, South Dakota on Thursday evening, briefly interrupting the proceedings meant to mark the 250th anniversary of the United States. The weather passed. Donald Trump's argument did not.

Standing below the granite faces of Washington, Jefferson, Roosevelt, and Lincoln, Trump delivered what began as a sweeping celebration of American exceptionalism before hardening into a near-half-hour political warning that would not have sounded out of place in the early 1950s. He declared communism not just a current threat but the single greatest menace the United States had ever confronted, greater in his explicit framing than the two World Wars, Pearl Harbor, and September 11 combined. The A...