Republican Congressman Tom Kean Jr, who has not been seen in Washington or his New Jersey district for months, secured his party's nomination for re-election on Tuesday, after receiving a recent endorsement from President Donald Trump.

"Tom Kean has my Complete and Total Endorsement for Re-Election," Trump wrote on social media, adding, "HE WILL NEVER LET YOU DOWN!"

Kean ran unopposed in the Republican primary and did not need to campaign for votes. But US media report he also has been absent from Congress, missing more than 100 votes.

In November's midterm elections, Kean will face Democrat Rebecca Bennett, a former US Navy helicopter pilot, who won her party's primary on Tuesday.

Kean was last seen on 5 March. His staff said in April that he was dealing with a medical issue. But Kean's continued absence has become a national curiosity in the US.

In his endorsement posted to Truth Social on Monday, Trump wrote that Kean was a "Tremendous Advocate of our America First Agenda" and "is working tirelessly".

Shortly after his last vote in Congress in March, colleagues, Republican officials and news organisations reported concerns about his whereabouts. Text messages went unanswered, as did requests for interviews.

An aide told The New York Times, "There's no cameras where Tom is."

In the statement released by his office on 27 April, he said he expected to be back soon.

"My doctors continue to assure me that my recovery will be complete and that I will be back to the job I love very soon," Kean said in the statement, posted on X.

"I expect to return to a full schedule and be at 100 percent."

Nearly a month later, Kean participated in a telephone interview with the New Jersey Globe and reiterated his intent to run for re-election and return to public life.

"I understand the need for public transparency, and I appreciate the support of my constituents," he said.

"I anticipate that in the next couple of weeks, I'll return to voting and to the campaign trail."

As voters began casting ballots in the election on Tuesday, there still had been no reported sightings of Kean.

The BBC has left a message with his office seeking comment.

The district he represents is considered a swing district, meaning it has a high propensity to flip between Republicans and Democrats each election cycle.

His will be a key district for Republicans to hold on to in November's midterm elections, which will determine the balance of power in Congress. Democrats are looking to unseat him and shift power their way and deliver Trump a loss.

Trump's endorsement of Kean given the stakes is unsurprising. The US president also has not shied away from endorsing candidates, even as they court controversy - or mystery.