At age 81, the Vermont Democrat has served in the chamber for nearly half a century.WASHINGTON — 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 seek re-election in 2022, closing out nearly half a century of service in Congress.Mr. Leahy, 81, who was elected in 1974 at age 34 after working as a prosecutor, announced his decision at a news conference at the Vermont State House in Montpelier.“It is time to put down the gavel — it is time to pass the torch to the next Vermonter,” Mr. Leahy said, speaking in the same room where he announced his first campaign for the Senate. “It is time to come home.”Mr. Leahy’s departure is unlikely to alter the partisan makeup of the Senate given that Vermont is a solidly Democratic state; President Biden won with 65 percent of the vote there last year. But it will rob the chamber of an institutional figure who has come to embody its traditions and the comity that defined it in a bygone era.“It is hard to imagine the United States Senate without Patrick Leahy,” said Representative Peter Welch, the lone Vermont Democrat in the House, who is widely seen as a possible successor to the senator. “No one has served Vermont so faithfully, so constantly, so honestly and so fiercely as Patrick.”The only sitting senator who served during President Gerald R. Ford’s term, Mr. Leahy is the first and only registered Democrat to be elected to represent Vermont in the Senate. He recently became the fourth longest-serving senator in American history, and finishing his term at the beginning of 2023 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. He is also the chairman of the powerful Senate Appropriations Committee, which oversees government funding.That seniority meant that Mr. Leahy played a unique role in the second impeachment of former President Donald J. Trump. After fleeing the Senate chamber as rioters stormed the building during the certification of Mr. Biden’s victory, Mr. Leahy both presided over the trial and served as a juror, ultimately voting to convict Mr. Trump.In his news conference, Mr. Leahy reflected on the granular legislative work of more than four decades in the Senate. He spoke about the bills he shepherded through to support forestland and farmland in Vermont, increase nutrition assistance across the country and help the victims of the Vietnam War.And he spoke about the legislation and the judicial appointments — including every sitting member of the Supreme Court — that he oversaw as a member of the Senate Judiciary Committee and as the panel’s chairman. Mr. Leahy also gained prominence for his push to curtail domestic surveillance after the Sept. 11 terrorist strikes — work that led to him being targeted in the anthrax attacks on Capitol Hill.“Representing you in Washington is the greatest honor,” he said on Monday. “I’m humbled, and always will be, by your support. I’m confident in what the future holds.”“It has been such a great part of our lives,” he concluded, standing beside his wife, Marcelle, who could be seen dabbing her eyes as supporters applauded. His office circulated his remarks announcing the “decision he and Marcelle have made about 2022.”In a later interview, he described the walks through the fields and woods near their home that helped lead the pair to conclude “this was the right time.”“We want more time to be in Vermont, want more time with family,” Mr. Leahy said. His long-lasting tenure, he added, “was not my idea when I first ran,” but ultimately “was a pretty good arc.”Mr. Leahy has expressed mounting frustration in recent years with the gridlock that has stymied most legislation in Congress, often complaining to reporters during the tensest moments in negotiations that bills should just be put on the floor for an up-or-down vote. He is a lead sponsor of legislation to expand voting rights protections, which Republicans have blocked repeatedly this year through filibusters.While he has kept up a steady stream of work in the Senate, where the average age is about 64, Mr. Leahy’s health has faltered at times recently. In January, shortly after opening the impeachment proceedings, he was briefly taken to the hospital for observation after he reported not feeling well.“Very few in the history of the United States Senate can match the record of Patrick Leahy,” said Senator Chuck Schumer of New York, the majority leader. “He has been a guardian of Vermont and more rural states in the Senate, and has an unmatched fidelity to the Constitution and rule of law.”Mr. Schumer expressed confidence that the seat would remain in Democratic hands, though in an interview, he declined to single out any candidate and said he would wait for the voters to decide. Senator Bernie Sanders, 80, an independent who caucuses with Democrats, will become Vermont’s senior senator upon Mr. Leahy’s retirement.“He leaves a unique legacy that will be impossible to match,” Mr. Sanders said.Mr. Leahy’s announcement means that the top two positions on the Appropriations Committee will be vacated at the end of 2022. Senator Richard C. Shelby of Alabama, the panel’s top Republican and a close friend of Mr. Leahy’s, is also retiring.But Mr. Leahy vowed that he would oversee the completion of the dozen annual spending bills to keep the government funded. He joked on Monday that his approach at the helm of the Appropriations Committee “was simple: help all states in alphabetical order, starting with the letter V — Vermont.”“I’m sure he would agree one great way to honor him would be to make sure this year’s appropriations bills are signed into law as soon as possible,” Senator Patty Murray of Washington, another senior Democrat on the Appropriations panel, said on Twitter.Born almost blind in one eye, Mr. Leahy found photography to be an outlet and is known to carry a camera on Capitol Hill, snapping photos of both his colleagues and the journalists who roam the hallways. (He is also working on a memoir that will reflect his bipartisan work and his photography.)Outside the Capitol, Mr. Leahy is known as a fan of Batman, and he has made appearances in the franchise’s films, including confronting Heath Ledger’s Joker in “The Dark Knight.”He also takes great pride in the prized real estate that comes with being the most senior senator, often taking visitors, reporters and colleagues to his private hideaway office in the Capitol to marvel at the breathtaking views of the Washington Monument and the National Mall. More