Thomas Kean

The path to a House majority runs straight through swing districts like New Jersey’s Seventh, which stretches the width of the state and includes wealthy suburban towns and rural conservative bastions.

Party control of the seat there has flipped back and forth twice in the last decade, making the November election by far the most competitive House race in New Jersey.

Four Democrats are competing in Tuesday’s primary for the chance to run against the incumbent, Representative Thomas Kean Jr., in the high-stakes midterm contest.

But it is Mr. Kean, a Republican who has not been seen publicly in Washington or in New Jersey for nearly three months, who has managed to overshadow the debate. On Monday night, even President Trump weighed in, claiming that Mr. Kean has been “working tirelessly” on behalf of his agenda.

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

Mr. Kean, 57, has missed more than 100 floor votes since March while being treated for what his aides have described only as a “personal medical condition.”

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