Senator Richard J. Durbin of Illinois, the No. 2 Senate Democrat for two decades and a leading liberal voice on Capitol Hill, announced on Wednesday that he would not seek re-election next year, closing out a 44-year congressional career focused on immigration, the federal justice system and anti-smoking initiatives.
The decision by Mr. Durbin, 80, was widely expected and will immediately touch off a crowded competition for a rare Senate vacancy in his solidly blue state. It also intensifies a generational shift in the chamber as he becomes the fifth sitting senator to announce a retirement, all of them over the age of 65.
In an interview ahead of his announcement, Mr. Durbin, who is in his fifth Senate term, said it was not an easy choice to step away from his prime perch doing battle with President Trump, whom he considers a dire threat to democracy. He described moments in recent days “where I thought, ‘Man, I don’t want to miss this fight.’”
“But you know,” he continued, “I have to be honest about this. There are good people in the wings, good people on the bench ready to serve, and they can fight this fight just as effectively as I can. There comes a point where you have to face reality that this is the time to leave for me.”
“In my heart,” the senator said in a video announcing his decision, “I know it’s time to pass the torch.”
Several Illinois Democrats have indicated an interest in running if the seat opened up and have been readying for a potential candidacy. They include Representatives Lauren Underwood, 38; Raja Krishnamoorthi, 51; and Robin Kelly, 68, along with Lt. Gov. Juliana Stratton, who is 59.
It is likely to be one of several highly competitive primaries in both parties over the next 18 months, as Democrats embark on an uphill slog to reclaim the Senate majority and Republicans grasp to hang on to it.
Mr. Durbin’s departure will reverberate in the Senate, where he played a prominent role on a host of big issues and was considered a forceful advocate of party positions. He was among the first to push for a path to citizenship for some undocumented immigrants brought to the United States illegally as children, a group that came to be known as Dreamers that became the focal point of successive failed efforts to overhaul immigration laws.
His retirement also will open up a top Democratic leadership position for the first time in a decade.
Mr. Durbin entered Congress in 1983 after narrowly defeating an 11-term Republican incumbent in a newly redrawn Springfield-area House district more favorable to Democrats. As a junior member of that chamber, Mr. Durbin led the drive to ban smoking on airplanes, helping to usher in the smoke-free movement with legislation signed by President Ronald Reagan in 1988.
“That has to be the most significant thing I’ve done in terms of changing America,” Mr. Durbin said. “I was trying to get away from a health hazard, and I ended up reaching a tipping point on tobacco in America. I didn’t see that coming.”
He won his Senate seat in 1996 after his mentor and friend Paul Simon retired and endorsed Mr. Durbin to succeed him, a significant boost for an Illinois politician from outside the Chicago area. Mr. Durbin is now his state’s longest-serving popularly elected senator.
A longtime force on the Judiciary Committee, Mr. Durbin became its chairman in 2021, resisting calls from progressives that he make room for a younger member given his leadership role and other legislative responsibilities.
He subsequently oversaw the confirmation of 235 federal judges during the Biden administration — including one Supreme Court justice — surpassing by one the number of judges confirmed by Republicans during an aggressive push in Mr. Trump’s first term. Mr. Durbin also brushed aside suggestions that he abandon the committee’s so-called blue slip tradition, which gives senators veto power over judiciary and law enforcement picks for their states, a lever that Democrats are now employing to thwart Mr. Trump.
After the 2004 election, Mr. Durbin was encouraged by Senator Harry Reid of Nevada, then the new Democratic leader, to seek the party’s second-ranking slot, known as the party whip. A few years later, Senator Chuck Schumer of New York became the third-ranking party leader, allowing Senate Democrats to take advantage of his fund-raising and strategic skills after he managed the party’s campaign arm.
The three worked closely at the top of the party, creating a sensitive situation when Mr. Reid announced in 2015 that he would not run again. He backed Mr. Schumer for the top job over Mr. Durbin, who had roomed with Mr. Schumer in a Capitol Hill rowhouse. Mr. Durbin also supported Mr. Schumer for the top post and held on to his own leadership job, resulting in his becoming one of the longest-serving party leaders in congressional history.
In a statement, Mr. Schumer joined numerous Senate Democrats in praising Mr. Durbin, calling him “a trusted partner, one of the most respected voices in the Senate for decades.” Senator Brian Schatz, Democrat of Hawaii and one of those interested in succeeding Mr. Durbin in leadership, called him a “titan of the Senate.”
“For more than four decades, Senator Dick Durbin has been a pillar of leadership, integrity, and unwavering dedication to the people of his home state of Illinois and the nation,” Mr. Schatz said in a statement.
Mr. Durbin is one of the remaining senators who relishes a chance to mix it up on the floor with lawmakers across the aisle. He has also forged close relationships with some Republicans, including Senator Charles E. Grassley of Iowa, who has been chairman of the Judiciary Committee in periods when Mr. Durbin was the top Democrat.
The two pushed a criminal justice overhaul that overcame Republican resistance to be signed into law by Mr. Trump during his first term, leading to the early release of thousands of prisoners who had their sentences recalculated. In celebrating the sixth anniversary of that law, the First Step Act, in December, Mr. Durbin noted that less than 10 percent of the 40,000 prisoners released under the law, many of them drug offenders, had been rearrested or jailed, far less than the typical recidivism rate.
“It’s no secret I enjoy working w Senator Durbin,” Mr. Grassley, 91, said in a social media post on Wednesday, adding that he would be “sorry” to see his Democratic colleague retire. “Obviously he’s liberal & I’m conservative,” but that did not stop them from having “a wonderful working relationship,” as reflected in their work on the First Step Act, the Republican wrote.
Mr. Durbin was an early critic of the war in Iraq, voting against the 2002 authorization for military force. He criticized the mistreatment of detainees at the U.S. military prison in Guantánamo Bay, Cuba, as comparable to the acts of “Nazis, Soviets in their gulags, or some mad regime — Pol Pot or others — that had no concern for human beings” and later apologized for a “poor choice of words.”
In 2001, he introduced the Dream Act with Senator Orrin G. Hatch, Republican of Utah, in an effort to create a path to citizenship for younger undocumented immigrants who had been brought to the United States as children. It never became law, but it laid the foundation for the Deferred Action for Childhood Arrivals program, which was announced by President Barack Obama in 2012 and protected hundreds of thousands of immigrants from deportation. Mr. Trump has tried to end the program, and it remains under legal threat.
“It’s been a long and challenging decision process,” Mr. Durbin said about retiring. “But I’ve been around the Senate long enough to realize that whatever your area of major interest in the Senate, you’re always going to minor in aging.”
Nick Corasaniti contributed reporting from New York.
