Take what happened in December 2012, at the height of negotiations over the “fiscal cliff.”

One day, congressional leaders went to the White House to meet with the president. As they entered, Secret Service agents decided to screen staff members, who usually roll right onto the grounds with their bosses. According to a person familiar with the day’s events, Mr. Krone, incredulous, began shouting. He then called Ms. Mastromonaco, then his fiancée and the administration’s deputy chief of staff for operations, who arrived and apologized. (Mr. Krone said he did not recall the incident and suggested that he might have been misunderstood. “I have a sarcastic sense of humor,” he said.)

Adding to the tumult as the staff members and congressional leaders waited in the White House lobby, Mr. Boehner approached Mr. Reid and, upset by Mr. Reid’s attacks on him on the Senate floor, told him to “go [expletive] yourself.” Mr. Reid replied that he read only what Mr. Krone put in his speeches.

“He says, ‘Blame David,’ ” Mr. Krone recalled, chuckling. “And I was, like, ‘Don’t look at me!’ ”

It is hard to imagine now, but Mr. Krone used to have a good relationship with the White House. Smart and insanely hard-working, Mr. Krone, with his direct manner and total empowerment by Mr. Reid, proved a valuable ally in the administration’s early policy lifts. He called himself an Obama “shill” for health care legislation and worked in the budgetary trenches with the White House’s chief congressional liaison, Rob Nabors. He began dating Ms. Mastromonaco and, in 2013, even golfed with the president on one occasion and, on another, played eight holes with his chief of staff, Denis R. McDonough, though that relationship later fell apart too.

“It was Denis,” Mr. Krone said, blaming Mr. McDonough for accusing him of leaking. After listening to the president tell Mr. Reid as much on the call, Mr. Krone seemed to vanish from Capitol Hill. Asked what happened, Mr. Krone said that splitting headaches he began suffering in 2012 had returned.

He called on his friend Mr. Nabors to help Mr. Reid in his absence. That inspired deep gratitude in Mr. Krone, and when Mr. Nabors faced his own personal setbacks, Mr. Krone said he was dismayed with the harsh way Mr. McDonough treated him. “You can’t discard people like that,” he said, adding, “and to be honest with you, I think they miss Rob.”

The White House declined to comment.

Even as his relationship with the administration deteriorated, Mr. Krone set a wedding date with Ms. Mastromonaco for last November. As the big day approached, Mr. Krone’s good friend George E. Norcross III, the Democratic political boss of South Jersey, suggested a golf outing at his Palm Beach, Fla., home before the nuptials. Mr. Krone said his fiancée endorsed the idea, but a week before the trip said, “Don’t get mad, but they are throwing a party for us.” The “they” in question was Mr. Obama and the first lady, Michelle Obama, but Mr. Krone kept his engagement with Mr. Norcross instead. “I’m exactly where I wanted,” he recalled thinking during the Florida trip.

At the White House engagement party, the president spoke of Ms. Mastromonaco’s indispensability and referred to her as a “little sister.” Michelle Obama declared her to be like “part of my family.” The absent groom later admired a photo of the cake served at the party, describing it as “like taller than me.”