In his own self-interest, Mr. Trump should be working overtime to prevent a third-party challenge that would attract Republicans and make his defeat likely. The more Mr. Trump declares war on party dissenters, the more Republicans he will convince that the Presidency is probably lost in any case and even a long-shot third-party campaign may be worth the political risk.

Above all, now that Mr. Trump is the presumptive nominee, millions of Americans are looking at him as a potential President. They are watching with a new seriousness and making judgments about his character and political values that will be hard to change once they become fixed. This audience of voters is far larger than the 10.6 million who have supported him in the primaries.

If Mr. Trump really wants to be President, as opposed to losing as a populist martyr, he has to build a broader coalition. He needs to persuade doubters not by lashing out but by sounding like a potential President. He needs to find a running mate people will respect and advisers willing to serve in his Administration. And he needs to give thoughtful speeches with specific ideas for reviving economic growth and keeping the country safe.