President Barack Obama called Harry Reid, the Senate majority leader, to broach a particularly delicate subject. It was during last year’s government shutdown and standoff with Republicans, but Obama’s frustration focused on one of their own. The president said he suspected David Krone, Reid’s intensely loyal and influential top aide, of leaking to the news media and requested that he stay away from future meetings.

It did not take much time for the president’s comments to reach Reid’s right-hand man. To Obama’s surprise, Krone was listening in on the call. Suddenly, the aide piped up and made it clear to the president that he did not appreciate the accusation.

On Friday, Obama and Reid traveled to Nevada, a trip that allowed the president to promote his executive action on immigration and the senator to unofficially start his re-election campaign. The public harmony contrasts with the private discord between the two men and their staffs, much of which has centered on Krone, a wealthy former cable executive whose defining principle is to defend Reid at all costs.

Administration and congressional officials have argued that the transgressions of Krone — publicly challenging the president, betraying the Oval Office code of silence and acting more like a senator than a staff member to one — have damaged Democratic unity at a time when the party can least afford it, as its numbers in Congress dwindle and the president sorely needs discipline in his ranks to advance what is left of his agenda.

It all exploded this month when Krone, 48, astonished even some veteran Capitol Hill staffers by speaking on the record in a highly critical way about private conversations with Obama.

Krone said he was simply protecting Reid. A few days before the midterm elections, he said, he was hearing from reporters that the White House was blaming the legislative strategy devised by him and Reid for the party’s lousy electoral prospects. "I’m going to go meet with these reporters," Krone recalled telling Reid. "And he’s, like, ‘OK.’" Krone then debriefed Washington Post reporters about an Oval Office meeting in which Reid pleaded in vain with Obama to do more to help endangered Democrats, suggesting that the president was to blame for the party’s disastrous showing. The article, which was published the day after the election, landed like an atom bomb on official Washington.

"Undercutting the president’s staff at a time of transition to a new majority is pretty outrageous," said William M. Daley, a former White House chief of staff. "For Krone to do this and there’s no retribution? Unbelievable." Aides in Speaker John A. Boehner’s office, who avoid emailing Krone because of an earlier breach, said the aide has "burned both sides of the aisle." The White House has banned all congressional staff members from attending White House meetings in an apparent effort to keep Krone out. ("We know that that’s because of me and I don’t really care — that’s fine," Krone said.)

Krone’s wife, Alyssa Mastromonaco, a former West Wing staff member who is extremely close to the president, was upset that he publicly criticized her former colleagues, Krone said, but understood that he needed to take action. She did, however, draw the line on letting her husband, an assiduous note taker, ever write a book about the White House. "She said they probably hate her already," he explained.

One person who did not seem bothered at all was Reid. Without much conviction, the senator, 74, said he did not ask Krone to hammer the president for him, but also did not regret him doing so. "He didn’t make it up, you know," he said. Asked if he worried that the escalation had damaged the ability of Senate Democrats to work with the administration, Reid raised his soft voice. "They should just get over it. I have a good relationship with the president. This is all staff driven. Get a life. Forget about this," he said. He has apparently already forgotten Obama’s unusual request to keep Krone out of meetings.

"I don’t remember anything about that," Reid said in his chandeliered office on Nov. 13, a few hours after being re-elected leader of the Senate Democrats. "Do you?" he asked, turning to Krone, seated beside him in the "leader’s chair."

"Umm," Krone, who is rarely at a loss for words, said through a frozen smile. A few minutes later, Krone, dressed impeccably in one of his bespoke suits, walked a reporter out of the office, and, referring to the president’s call, jocularly exclaimed, "I can’t believe that you know that story."

Krone first met Reid in 1992 when, as a young cable television executive, he picked the senator up from the airport for a fundraiser in Colorado. Over the next several years, he donated to Reid’s campaigns, befriended his finance director and, the senator said, grew close to the women on his staff. By 2003, when Krone took a top job at the National Cable and Telecommunications Association in Washington, the two had become close enough for Reid to suggest that Krone, by then a millionaire, take an apartment in the Ritz-Carlton, where the senator lived. The Mormon senator from Searchlight and the Jewish executive from Philadelphia bonded.

"I don’t smoke and I don’t drink, and it’s not — I mean with him it’s obviously religious and deep feelings," Krone, who has climbed Everest, said. "For me it was just who I am."

For some on Capitol Hill, Krone is a manipulative megalomaniac. For others, he is a hero who has the financial independence to speak his mind. The one thing that everyone agrees on is that he is different.

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 staffers, who usually roll right onto the grounds with their bosses. According to a person familiar with the day’s events, Krone, incredulous, began shouting. He then called Mastromonaco, then his fiancée and the administration’s deputy chief of staff for operations, who arrived and apologized. (Krone said he did not remember the incident and suggested his tone might have been misinterpreted. "I have a sarcastic sense of humor," he said.)

Adding to the tumult as the staffers and congressional leaders waited in the White House lobby, Boehner approached Reid, and, upset about Reid’s attacks on him on the Senate floor, told him to "go (expletive) yourself." Reid responded that he read only what Krone put in his speeches.

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

It is hard to imagine now, but Krone used to have a good relationship with the White House. Smart and insanely hardworking, Krone, with his direct manner and total empowerment by 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 Mastromonaco, and, in 2013, even golfed with the president on one occasion, and, on another, played eight holes with his chief of staff, Denis McDonough, though that relationship later fell apart, too.

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

He called on his friend, Nabors, to help Reid in his absence. That inspired deep gratitude in Krone, and when Nabors faced his own personal setbacks, Krone said he was dismayed with the harsh way 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, Krone set a wedding date with Mastromonaco for last November. As the big day approached, 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. 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 Obama and the first lady, Michelle Obama, but Krone kept his engagement with 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 Mastromonaco’s indispensability and referred to her as a "little sister." Michelle Obama declared that the bride-to-be was 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."

On Nov. 22, 2013, Supreme Court Justice Elena Kagan married the couple, taking an orchid from Clarence Thomas’ office to create a more festive atmosphere in her chambers. But the anger toward Krone in the West Wing had made Mastromonaco’s situation untenable.

Krone acknowledged that his battles with the White House and his recent speaking out had put his wife "in a tough spot." Mastromonaco, who declined to comment, this week announced she had taken a job at Vice, the Brooklyn-based media company, and the couple has now bought an apartment in Manhattan, furthering speculation that he will soon depart Reid’s office. But Krone scoffed at the notion that Reid would ask him to step aside.

"I don’t think Harry Reid would ever say that to me," he said. "I love the man."

The feeling is mutual. Reid fought back tears as he recalled the time he visited his wife, who had been injured in a car accident, and saw Krone at her hospital bedside. "David is someone I can say, and it doesn’t affect my manhood at all," Reid said. "I love David Krone."

Jason Horowitz, New York Times

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