Senator Patrick J. Leahy, Democrat of Vermont and the chamber’s longest-serving member, announced on Monday that he would retire at the end of his term rather than seeking re-election in 2022, closing out nearly half a century of service in Congress.

Mr. Leahy, first elected in 1974 at age 34 after working as a prosecutor, announced his decision at a news conference in Montpelier, Vermont’s capital.

“It is time to pass the torch to the next Vermonter,” Mr. Leahy, 81, said. “It is time to come home.”

The only sitting senator who served during President Gerald Ford’s term, Mr. Leahy is the first and only registered Democrat to be elected to represent Vermont in the Senate. Mr. Leahy recently became the fourth longest-serving senator in American history, according to Senate records, and finishing his term at the end of 2022 will make him the third longest-serving.

Mr. Leahy serves as president pro tempore of the Senate — a role reserved for the majority party’s most senior member that puts him third in line to the presidency — and the chairman of the powerful Senate Appropriations Committee, which oversees government funding.