A vulnerable Republican congressman who has not voted in weeks “is attending to a personal health matter”, the speaker of the House, Mike Johnson, said on Friday as he struggles to maintain his historically small majority in Congress’s lower chamber.

Tom Kean Jr’s New Jersey district is a top pickup target for Democrats in the November midterms, but the congressman has not cast a vote in the House since 5 March.

In a statement, Johnson said he had spoken to Kean, 57, on Thursday by phone, and said “he is attending to a personal health matter and expects to be back to 100% very soon. Tom is one of the most dedicated and hardest-working members of Congress, and I am grateful for all he does and will continue to do to serve New Jerseyans and our country.”

Harrison Neely, a spokesman for Kean’s campaign, said: “The congressman is handling a personal health matter. He will be 100% healthy and back to a full schedule soon.” He did not respond to specific questions about Kean’s health and when he stopped working.

Kean’s absence is the latest complication for the historically small Republican majority in the House, which has been rocked by a spate of resignations and deaths by lawmakers of both parties.

This month, Democrat Eric Swalwell of California resigned amid allegations of sexual assault, and Republican Tony Gonzales of Texas followed suit after admitting to an affair with an aide who later took her own life.

A week later, Democrat Sheila Cherfilus-McCormick of Florida resigned ahead of a potential expulsion vote after she was indicted on charges related to allegedly misusing $5m in federal disaster relief funds.

Two more seats are vacant after the deaths of California Republican Doug LaMalfa and Georgia Democrat David Scott this week.

The Republican majority stands at 217 seats plus one independent who caucuses with them, to the Democrats’ 212 seats. The five vacancies can nonetheless create attendance challenges for Johnson on party line votes in the chamber, which is in the midst of prolonged negotiations over issues such as funding for the Department of Homeland Security and a contentious surveillance law.

A member of a prominent New Jersey political family whose father served as governor and grandfather was a longtime congressman, Kean beat Democrat Tom Malinowski in 2022 to win a seat representing a district in the state’s north-west corner, on the Pennsylvania border.

Kean won re-election in 2024 as Donald Trump carried his district by a narrow 50%. But with polls showing Democratic candidates gaining in popularity nationwide, Kean is viewed as the most vulnerable Republican congressman in New Jersey, and leftwing groups have announced six-figure ad buys intended to convince voters to unseat him.