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.”

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Mr. Durbin with Senator Jon Ossoff of Georgia, the chamber’s youngest member. “There are good people in the wings,” Mr. Durbin said, “good people on the bench ready to serve.”Credit…Haiyun Jiang/The New York Times