WASHINGTON (AP) — On one of the worst days of his presidency, Donald Trump was chatting aboard Air Force One when the conversation took a detour into gallows humor.

Trump was returning from a rally in West Virginia just hours after two former members of his inner circle were found or pleaded guilty, when one passenger quipped that a news story would surely soon be breaking about the president fuming onboard. Everyone laughed, including the president.

Despite the momentary levity, though, Trump is increasingly frustrated and isolated as the investigations that have long dogged his White House p...