Where is Representative Thomas Kean Jr.?

Lately, curious political watchers have noted where Mr. Kean is not. He seems to be absent on Capitol Hill, where he represents the people of New Jersey’s Seventh District but hasn’t cast a vote in more than two months. Mr. Kean, a Republican, has not been seen on the campaign trail as he runs for re-election in a competitive midterm race.

He has not appeared in pixel form, either: No candid photos have emerged of the incumbent to allay the worries of donors and constituents. Two weeks ago his office released a written statement, attributed to Mr. Kean, explaining that he was dealing with a “medical issue” and would be back “very soon.”

Since then? No reported sightings.

Mr. Kean’s social media accounts have remained active in his prolonged absence. The posts are often written in the first person, but paired with older photos.

“What we are being assured is that his team is carrying the torch,” Tina Shah, who is vying for the chance to unseat Mr. Kean, said on Tuesday at a debate with three other Democrats. “But we elected Tom Kean Jr., not his team.”

Members of Mr. Kean’s team were seated conspicuously in the third row, watching as the Democrats took turns weighing in on their boss’s mysterious absence. Afterward, his chief of staff, Dan Scharfenberger, reiterated a now familiar explanation.

“He’s dealing with a personal health condition, and he’ll be back soon,” Mr. Scharfenberger said in an interview.

When pressed on the nature of the health condition and why Mr. Kean had not been seen in New Jersey or in Washington, Mr. Scharfenberger said, “There’s no cameras where Tom is.”

He declined to elaborate.

National Democratic officials see Mr. Kean’s district — a largely affluent and suburban region that stretches like a girdle across North Jersey from its eastern shoreline to its western border with Pennsylvania — as one of the party’s best pickup opportunities in November. And Democrats had been aggressively targeting his seat well before Mr. Kean began missing votes.

“He’s going to be totally fine, and he’ll be back to a full schedule soon,” Harrison Neely, Mr. Kean’s spokesman, said on Tuesday.

Soon may not be imminent. The Morris County Chamber of Commerce has amended the program for a May 28 breakfast meeting focused on Washington policies, saying that Mr. Kean will no longer be speaking at the event.

“As you no doubt have heard, Congressman Kean will be unable to appear due to his ongoing health concerns,” a notice posted on the chamber’s website states. “We wish him well.”

Mr. Neely had no immediate response on whether the congressman had canceled other similar events.

Democrats vying for the Seventh District seat have seemed cautious about attacking Mr. Kean, perhaps aware of the political risk of going after a man whose condition is not known. But at Tuesday’s debate, a few of them offered some measured criticism about the Republican’s lack of transparency.

“Look, at the end of the day you’re a public servant,” said Brian Varela, who owns child care centers and is campaigning to abolish the Immigration and Customs Enforcement agency. “And while I don’t think you need to be putting your own personal medical history out there, you at the very least have a responsibility to be communicating with your constituents and with your district.”

Michael Roth, who helped to lead the Small Business Administration during the Biden administration, said most workers would be required to be more transparent with their employers if they needed to take two months off.

“If you were missing work, you would tell your boss,” he said. “And Tom Kean Jr.’s boss is the people.”

Rebecca Bennett, a former Navy helicopter pilot who has worked as a strategist for health care companies, avoided discussing Mr. Kean’s absence and instead said the election was about the job Mr. Kean had been doing before he disappeared from public view.

Mr. Kean is the only Republican on the June 2 primary ballot. If he had exited the race at least 62 days before the primary, a three-person team of campaign advisers would have filled the vacancy. Republican leaders from the district’s six counties would select a nominee if he were to drop out after the primary. His aides maintain that the congressman has no plans to bow out.

Leonard Lance, a moderate Republican who lost re-election in the Seventh District in 2018 after five terms in Congress, said the loss of any of the state’s three G.O.P. House members would be a blow to New Jersey’s historically centrist core. But he said he believed Mr. Kean was still the best candidate “by far” to fend off what is likely to be a fierce Democratic challenge.

“I take at face value the statement of his office that he will be back at 100 percent,” Mr. Lance said.

There were no reported sightings of Mr. Kean on Wednesday, the day after the debate. However, the congressman’s X account did post an update on his activities.

“I’m pleased to join the Congressional Crypto Caucus,” the post said. “The United States must lead on digital assets — driving innovation, delivering regulatory clarity and protecting consumers. I look forward to working with this bipartisan group of colleagues to advance that agenda.”

Kirsten Noyes contributed research.