Mr. Bannon is not the first staffer to believe the White House agenda must mirror his own. But no other aide in memory has had such grandiose or destructive plans for trying to remain in charge after being shown the door.

Mr. Trump is also engaged in the threat-fest against his own party. At an Arizona rally on Tuesday, he excoriated Senate Republicans for failing to replace ObamaCare—rather than expressing confidence that a health-reform bill would pass eventually. The president also dismissed Sen. Jeff Flake —a critic of Mr. Trump who faces re-election in 2018—by saying “no one knows who the hell he is.” Most pointedly, he failed to wish Sen. John McCain a speedy recovery from brain cancer. All superb ways to encourage support from a thin GOP Senate majority.

Memo to the White House: The worst way to strengthen a president is publicly to blame his difficulties on allies. The least effective way to pass an agenda is to threaten the president’s party in Congress.