AMNA NAWAZ: Well, primary voters in New Jersey last night also finalized the race for one of the nation's most competitive seats and one of the most unusual, because the incumbent Republican, Congressman Tom Kean Jr., hasn't been seen on the campaign trail or Capitol Hill for months.

As congressional correspondent Lisa Desjardins reports, his absence raises broader questions.

LISA DESJARDINS: Whoever else has come by congressman Tom Kean's Capitol Hill office, for the past three months, he was not one of them.

What do we know about where he is or not?

BENJAMIN HULAC, NJ Spotlight News: We really don't know anything.

LISA DESJARDINS: Benjamin Hulac is the Washington correspondent for NJ Spotlight.

Tell us the status of Congressman Kean right now.

BENJAMIN HULAC: Wish I could.

I'm not trying to be oblique, but he last voted in the House on March 5 and has not been seen in public since.

And the only thing I have gotten out of his office is that he has a personal medical issue.

LISA DESJARDINS: This was Kean's last known day in Washington.

He appeared at a committee hearing.

REP.

TOM KEAN (R-NJ): I agree with my colleagues to support this legislation.

LISA DESJARDINS: Kean waited until late April to issue a cryptic statement, writing that "A doctor forecast full recovery very soon."

I asked House Speaker Mike Johnson about this today.

REP.

MIKE JOHNSON (R-LA): As soon as he's ready to talk about that, you're all going to breathe a sigh of relief and say that makes sense.

What he's dealing with is something very common and not a big thing.

LISA DESJARDINS: Kean has been the longest, but not the only absence this year.

Democrat Frederica Wilson, who's retiring, has missed a month of votes, eventually saying she had eye surgery.

Republican Julia Letlow missed two weeks of votes as she campaigned for U.S.

Senate.

MAN: The yeas are 212.

The nays are 212.

The concurrent resolution is not adopted.

LISA DESJARDINS: This all led to a tie, a near-win for Democrats on a statement vote last month to end the war in Iran.

So far this year, the House has been in session 76 days.

Congressman Kean has missed more than half of that.

But many days, a dozen or more are absent in a one-or-two-vote-margin Congress.

REP.

CHUCK FLEISCHMANN (R-TN): As the legislation becomes more important, in my case, for appropriation bills, every vote counts.

REP.

MARK ALFORD (R-MO): I represent 772,047 individuals, Republicans, Democrats, independents, and they expect me to show up to my job.

REP.

RO KHANNA (D-CA): Voters, by and large, are very, very understanding.

What upsets them is when there's a lack of transparency.

ELLIE POWELL, University of Wisconsin-Madison: Now both parties need every single member to be there on every single vote.

LISA DESJARDINS: Ellie Powell of University of Wisconsin-Madison studies when members miss votes and points out that Congress used to issue internal penalties for missed votes.

And stronger local journalism was a factor.

ELLIE POWELL: Sort of in an earlier era of congressional history, where your local paper would post how many votes you had missed and it would be really embarrassing during a campaign, Congress has just sort of stopped caring.

LISA DESJARDINS: The public has started noticing.

Two years ago, The Dallas Express discovered congresswoman Kay Granger had been living in an assisted living facility.

She had not voted in Congress for six months.

The rules of the House require that members be present for votes unless they have a formal or necessary excuse.

They can put in for an official leave of absence, but rarely do.

House leaders can reprimand members with fines, but these days they never do.

DANIEL SCHUMAN, American Governance Institute: And what we're seeing now is that members are disappearing for weeks at a time and just they just don't seem to be involved in the process.

MAN: Our next witness is Daniel Schuman.

LISA DESJARDINS: Daniel Schuman directs the nonpartisan American Governance Institute and is expert in congressional accountability.

Reasons abound for absences.

Colorado's Brittany Pettersen missed votes to give birth.

Others like Thomas Massie or Chip Roy have missed votes while in competitive primaries.

But add to that something harder to track, motivation.

DANIEL SCHUMAN: This arises from like just sort of the member's sense of personal responsibility, but it also arises from institutional incentives in the chamber, where most of the laws are being written by leadership.

LISA DESJARDINS: Polling shows Americans' opinion of Capitol Hill is near historic lows.

And, as a branch, Congress is facing unprecedented challenges from President Trump.

While most members are showing up to work, the problem of those who aren't undermine its power.

DANIEL SCHUMAN: They should be engaged in lawmaking.

They should be engaged in debate.

LISA DESJARDINS: Kean was unopposed and won his primary last night.

He posted on social media that he plans to be back within weeks and then will be completely transparent about his condition.

But, for now, NJ Spotlight'S Benjamin Hulac is covering a campaign for Congress where the Republican nominee is absent.

BENJAMIN HULAC: And when you're absent, you just don't really give a yardstick to voters about how to judge you.

LISA DESJARDINS: And, in modern times, voters are the only ones who decide if absent members get penalized.

For the "PBS News Hour," I'm Lisa Desjardins.