Representative Thomas Kean Jr., a New Jersey Republican running for re-election in one of the country’s most competitive districts, finally explained on Tuesday why he had been missing from Congress and out of public view for nearly four months.

“Several months ago, due to health concerns, I entered the hospital for some testing,” he said.

“I was given the diagnosis of depression,” he added.

His five-minute speech on the House floor appeared heartfelt and addressed some of the questions surrounding his mysterious absence, yet left others unanswered.

He did not elaborate on why he had entered the hospital or where he had been treated. He did not discuss medication or long-term treatment needs. He did not explain why it had taken so long for doctors to clear him to return to work.

Mr. Kean spoke from a lectern on the House floor during a tightly choreographed period that takes place before voting starts. Members can speak for up to five minutes to a largely empty chamber, and reporters are not given an opportunity to ask follow-up questions. Mr. Kean did not hold a news conference and refused to respond to requests for additional information as he moved through the Capitol.

Mr. Kean, 57, had been out of public view from early March until last Wednesday, when he answered the front door of his Westfield home after a reporter for The New York Times spotted him from the street, standing in a brightly lit front room.

Here’s what we know, and what we don’t, following Mr. Kean’s long-awaited re-emergence:

Why didn’t he share his diagnosis earlier?

Mr. Kean acknowledged in the speech how common depression is, raising questions about why he had chosen not to share information about his diagnosis or his whereabouts before now.

He made a glancing reference on Tuesday to his well-known desire to avoid the spotlight, suggesting that was at least part of the reason. “I am a private person by nature,” he said.

Since being elected to the State Legislature a quarter-century ago, Mr. Kean has been known to frequently dodge questions from reporters and voters. He and his family own an estate on Fishers Island, a remote and hard-to-reach enclave off the coast of Connecticut. And when he and his wife bought a Jersey Shore vacation home, they shielded the purchase behind a limited liability corporation with a unique name: Rendezvous L.L.C.

He also suggested that the timeline for his return had changed along the way after doctors recommended inpatient treatment.

“When I said I hoped to return in a matter of weeks, I believed it,” he said, an apparent reference to the first statement his office released nearly two months after he last voted in the House, on March 5.

“Those were the best estimates that the doctors could provide,” he said, adding, “There is no timeline for healing. There is no time line for recovery. Only the work of getting better, one day at a time.”

Party leaders say they support him.

Few people were aware of what Mr. Kean planned to say before he said it.

That included the Republican National Committee, which is expected to pump millions of dollars into the fierce fight to hold Mr. Kean’s Seventh District seat. Democrats have nominated Rebecca Bennett, 39, a former U.S. Navy helicopter pilot, and have aggressively targeted the seat in a race that could tip the balance of power in Washington.

“We’ll have to see what he’s going to say,” an R.N.C. spokeswoman, Kristen Cianci, said Monday.

Still, she said the party was planning to back Mr. Kean, who ran unopposed in early June for the nomination. (By the time he began missing votes, on St. Patrick’s Day, it was already too late to replace him on the primary ballot.)

“We’re still very confident of this race going into November,” Ms. Cianci said.

Bill Palatucci, one of the state’s two representatives on the national committee and Mr. Kean’s longtime campaign lawyer, said he expected the public to respond sympathetically to news of the congressman’s treatment for depression.

“Tom was brave, genuine and transparent,” Mr. Palatucci said after the speech. “Every family in the district has had a member battle depression or a mental health issue.”

His re-election was never going to be easy.

Long before he abruptly disappeared from public life, Mr. Kean was expected to face an exceedingly tough race for a third term.

Ms. Bennett, the Democratic nominee, has been careful not to sound uncaring as she discussed Mr. Kean’s absence and has instead focused on his voting record and his support for President Trump, whose approval ratings have sunk to historic lows, particularly in affluent suburban districts like Mr. Kean’s.

On Tuesday, Ms. Bennett subtly reminded voters of the candidates’ age difference and her success in a male-dominated branch of the military.

“I spent my life serving this country in uniform,” she said in a statement. “It’s time we hold Tom Kean Jr. accountable for his record and bring a new generation of leadership to Washington.”

Registered Republicans outnumber Democrats in the district by about 19,000 voters. And Mr. Kean’s refusal to hold in-person town halls did not stop him from winning a second term by a five-point margin in 2024.

But Jacob Rubashkin, deputy editor for Inside Elections, a nonpartisan newsletter, said Mr. Kean’s unexplained absence had reinforced an existing political vulnerability.

“This broke through in a way in a way that the Democratic bellyaching about his refusal to hold town halls or talk to the press didn’t,” Mr. Rubashkin said. Three weeks ago, Inside Elections shifted its race rating for the seat into its “tilt Democratic” category.

What do voters think?

Bill Morrissey, a lifelong Republican, reluctantly cast a ballot for Mr. Kean in the primary, concerned by the congressman’s lengthy absence and his fealty to Mr. Trump.

Mr. Morrissey, who lives close to Mr. Kean in Westfield, said he found the speech “heartfelt.” But he said he doubted it would lay to rest the tough questions about Mr. Kean’s ability to effectively serve his constituents.

“Unlike most physical ailments, which may elicit a degree of sympathy, depression still bears a stigma,” Mr. Morrissey, 76, said.

Joe LaBarbera, the Republican Party chairman in Sussex County, a conservative bastion in the northwest part of the Seventh District, commended Mr. Kean for confronting that stigma. Mr. LaBarbera said he had struggled with post-traumatic stress disorder after serving five combat missions in the U.S. Army.

Mr. Kean’s willingness to seek treatment was “a sign of real leadership,” Mr. LaBarbera said. “I wish I would have shown that courage after my many deployments.”