That duality in Mr. Trump’s behavior — acting as a bystander while other leaders answered a crisis and simultaneously raging at Republicans who have inched away from him — also amounts to a preview of Mr. Trump’s post-presidency.

He has shown no interest in shaping the debates that lay ahead for Republicans, in tending to the party’s electoral health or in becoming a champion of America’s recovery. Rather, he seems intent on using his political platform to wage personal vendettas and stoke a shared sense of grievance with the voters he has long cultivated as a fan base.

Senator Mitt Romney, the Utah Republican, said the president’s fury blinded him to his last best chance to buff his legacy: visiting vaccine distribution sites and clinics to highlight the possibility of hope after nine months of national misery.

“The president could have made that the hallmark of his last days in office,” Mr. Romney said. “Instead, he’s seen as promoting conspiracy theories and evidence-free accusations of fraud, which lead to a color of a sore loser.”