Presumptive Republican presidential nominee Donald Trump speaks during a campaign rally in Greensboro, N.C., on June 14. (Chuck Burton/Associated Press)

WHEN MITT Romney ran for the Republican presidential nomination four years ago, he carried a three-legged stool to signify the unity of the three traditional elements of the party: “economic conservatives, social conservatives and defense conservatives.” The GOP, he argued, must rest on a balance of these three, which made for a more or less comfortable peace behind his candidacy.

Four years later, a disunified party has tossed away the stool. After a brief moment of near-chaos stoked by the squelching of an anti-Trump minority on the Republican National Convention floor Monday, the party formally adopted a platform that reflects the party’s accelerating ideological confusion — and its lurching away from the center of American politics.

The party embraced a startlingly backward-looking social agenda, much of it a reaction to the Obergefell v. Hodges Supreme Court ruling that legalized same-sex marriage. The party favors a constitutional amendment that would overturn the ruling, enabling states to ban same-sex unions once again. It proclaims that children should be raised by “a mother and father,” expressing disapproval of gays and lesbians as parents. It signals approval for the discredited practice of gay conversion therapy. It calls for the Bible to be taught in public schools. It supports allowing churches to organize politically while keeping their tax-exempt status.

The party’s allegiance to tax cutting and deregulation remains. Among other things, the GOP would curtail a variety of environmental protections and prevent a variety of species from being listed as endangered. It would abolish the Internal Revenue Service, a particularly silly bumper-sticker proposal.

But Donald Trump’s takeover has moved the GOP toward isolationism, anti-trade populism and a concomitant ambivalence on essential economic freedoms. The platform says “we need better negotiated trade agreements that put America first” — which implies criticism of past trade deals that Republicans have traditionally defended and projects opposition to future trade pacts. The platform takes no firm stand on the North American Free Trade Agreement or the Trans-Pacific Partnership. This is a shocking turn for a party whose leadership prioritized fast-tracking the TPP after its 2014 midterm election victory.

Filling out the picture, the platform adopts Mr. Trump’s call to build a wall on the U.S.-Mexico border, which would be an expensive (if ineffective) symbol of America shutting out the outside world. Where Republicans just four years ago proclaimed that they “will lift the torch of freedom and democracy to inspire all those who would be free,” Trump forces nixed a line from this year’s platform that called for arming Ukraine against Russian aggression.

The net result is a platform that is more reactionary than visionary, with an emphasis on social matters that is out of step with American public opinion and an isolationist turn that reeks of counterproductive nativism. Party platforms, it is often said, are irrelevant. In this case, the nation can only hope so.