Representative Tom Kean Jr., the New Jersey Republican who disappeared from Congress and the campaign trail in March with almost no explanation, is set to return to the Capitol on Tuesday and address the mysterious health condition he says has kept him away.

Mr. Kean, a 57-year-old seeking a third term in a competitive district, has missed more than 100 votes since he was last seen in public more than 100 days ago. His re-emergence will be closely watched after months during which he and his staff refused to disclose anything about where he was or what was keeping him away.

Their silence has built Tuesday’s return into a major reveal after a prolonged cliffhanger. Even last week, when a reporter for The New York Times found Mr. Kean at his home in Westfield dressed in a suit and tie around 8:45 p.m., he declined to offer any explanation, saying: “I’ll talk to you next week.”

In the absence of official information, his own colleagues have speculated wildly about Mr. Kean’s condition, privately raising an array of possibilities for his long and unexplained absence.

Could it be rehab for a stroke, heart condition or addiction issue? Was it a case of plastic surgery gone awry? Might he reappear on Capitol Hill as a woman? (His brief appearance at home last week put at least some of that speculation to rest.)

Mr. Kean has said only that he is dealing with a “personal medical issue,” and until recently offered no timeline on his return, only vague assurances that when he did come back, he would be fully recovered and transparent about what he had been through.

Mr. Kean’s office also did not provide any detail on Monday about his return, though CNN reported that he planned to give a speech on the House floor. He also was scheduled to participate in a fund-raising reception on Tuesday evening in Washington, according to an email obtained by The Times that confirmed an earlier report by Politico.

His chief of staff, Dan Scharfenberger, did not respond to calls and emails about the congressman’s planned schedule for Tuesday. A spokesman for Speaker Mike Johnson said they were leaving the details of Mr. Kean’s return to him.

Mr. Kean has invited some Republican officials to participate in a 2 p.m. conference call on Tuesday, according to multiple people who were invited. The people said they expected he would have to address the health issue in some way.

With an election in five months, Mr. Kean’s months of mysterious silence have tested the limits of what the public will tolerate in terms of privacy for its leaders.

Presidents traditionally release the results of their annual physicals and disclose what medications they are taking, although they are not legally required to do so. But members of Congress typically provide no information to the public about the state of their health or their fitness to fulfill their duties.

Voters tend to be forgiving about the health ailments of their leaders. And some lawmakers in the past have tried to turn their own medical challenges or experiences with mental health, alcohol or addiction into a way to relate to voters who may be struggling themselves.

So Mr. Kean’s decision to keep his constituents and his colleagues in the dark for so long has largely been viewed as inexplicable.

Tracey Tully contributed reporting.