On Tuesday night, President Trump will deliver a prime-time address to the nation “on the Humanitarian and National Security crisis on our Southern border.” It will be his first big address from the Oval Office, a signal that the president recognizes, at least on this issue, that to succeed he must reach beyond his base to rally the American people. He further promises to travel to the border Thursday to “meet with those on the front lines.”

The Oval Office speech and trip to the border suggest the stakes are rising as the...