Dingell, 87, told the Detroit Free Press that he’d reached the decision to retire at the end of his current term, his 29th full one, rather than run for re-election because it was time, given a list of achievements that any other member of Congress would envy, and his continued frustration over partisan gridlock.

It comes at a time when many members of both parties are moving toward the exits in both the House and Senate. Michigan is not only losing Dingell but U.S. Sen. Carl Levin, a widely respected Democrat with 35 years’ experience, who announced his retirement last year.

For weeks, rumors had circulated that Dingell, who last June surpassed the late U.S. Sen. Robert C. Byrd of West Virginia as the longest-serving member of Congress ever, might be considering retirement. While clearly sharp mentally, he could be seen in recent months peppering witnesses with questions before his beloved Energy and Commerce Committee, time has taken its toll on his body, forcing him to use crutches or a wheelchair to get around.

But less than two weeks ago, his office seemed to put those rumors to rest with an e-mail to constituents in which Dingell vowed to fight on for extended unemployment benefits and “to protect the many workers and industries important to southeast Michigan.” In the e-mail, he said he would “continue to reiterate to my colleagues that the words ‘compromise’ and ‘conciliation’ should not be considered dirty words in Washington.”

Dingell was expected to let his staff know about the decision Monday morning and then announce it publicly at a noon luncheon at the Southern Wayne County Chamber of Commerce in Southgate, Mich. Speculation will begin almost immediately about a successor.

Dingell’s wife, Deborah, who with her husband makes up one of Washington’s most prominent power couples, is widely considered a possible candidate; there has also been speculation that state Sen. Rebekah Warren of Ann Arbor could test the waters. It is considered a relatively safe Democratic district. Whoever replaces Dingell, she or he will have a tough act to follow.

John David Dingell Jr. was 29 years old when the Detroit native was elected in a 1955 special election to serve out the remainder of his late father’s term. Since then, he has cast tens of thousands of votes and helped pass, if not write, the most iconic pieces of legislation of the last six decades, from the Civil Rights Act and Medicare to the Clean Water Act, the Clean Air Act, the Endangered Species Act and, in 2010, the Affordable Care Act.

The man known throughout Washington as Big John, at 6-foot-3, he literally towered over witnesses before the House Energy and Commerce Committee, Dingell cut a distinctive figure in the Capitol. A progressive when it came to workers’ rights, he is also a staunch defender of Michigan industries, including its automakers, and at times ran afoul of environmentalists.

He also supported gun rights, and, as an avid outdoorsman, sat on the board of the National Rifle Association for a time, though he eventually quit. He counts as among his most important accomplishments the creation of the Detroit International Wildlife Refuge and the River Raisin Battlefield.

Gentlemanly and erudite, Dingell has always positioned himself as a centrist when it comes to getting legislation passed, looking for members on both sides of the political aisle to count on as allies. Dingell has decried the partisan atmosphere that has grown toxic in Washington, but he has managed to maintain friendships across the aisle, including with the current Energy and Commerce Committee chairman, U.S. Rep. Fred Upton, R-Mich. Dingell still likes referring to himself as “just a dumb Polish lawyer,” but his career has helped shape the way legislation is passed in Washington.

He vastly expanded the scope of the Energy and Commerce Committee’s purview during his first stint as chairman, which lasted from 1981 to 1995, to the point where it was said it handled four out of every 10 bills in the House. He used to have a photo of the earth from space behind his desk and when anyone asked him to define the committee’s jurisdiction, he’d point to it.

Dingell also became famous — or infamous, as the case may have been — among Washington bureaucrats, industrialists and others who came before his committees for oversight matters. He investigated the Environmental Protection Agency, the Food and Drug Administration and oversaw the breakup of AT&T, and his letters to agency officials, demanding specific answers to questions in a tight time frame, grew legendary as “Dingell-grams.”

Many felt Dingell might resign sometime after the 2008 election when, fresh off a knee surgery, U.S. Rep. Henry Waxman of California, a key ally of then-House Speaker Nancy Pelosi, took on Dingell for the Energy and Commerce chairmanship and won. But Dingell continued to work with Waxman to pass the ACA and has remained chairman emeritus and sat on all subcommittees.

Waxman, who had remained the ranking Democrat on the committee after Republicans took control of the House in 2011, announced his retirement from Congress in the last few weeks as well. With Waxman stepping down, there had been some speculation that Dingell might again ask his party to return him to the ranking member’s seat.

With Dingell’s departure at the end of the current term in early January, the longest-serving active member of Congress will be U.S. Rep. John Conyers, D-Mich., who once worked for Dingell and has been in the House since 1965.

Conyers, 84, would need 10 more years to match Dingell’s record for longevity.