WASHINGTON — Representative Bobby L. Rush, the most senior Illinois House lawmaker, said on Monday that he planned to retire at the end of the year, adding to a wave of Democrats who have decided against seeking re-election in what is expected to be a tough midterm cycle for the party.

Mr. Rush, 75, a pastor and former Black Panther who built himself into an electoral powerhouse in his district on the South Side of Chicago, said in an announcement video obtained by The New York Times that he wanted to focus on his ministerial work and his family.

“I’m not retiring — I’m returning home,” Mr. Rush said in the video, in which he reflected on nearly three decades of service in Congress and earlier battles with a rare cancer. “I’m returning to my church. I’m returning to my family.”

“Being a member of Congress — although it’s powerful — it can be very limited,” he added, saying he would be broadening his horizons. “I think I’ll be more effective because I have the gift of a 75-year-old, and that’s wisdom.”

In an interview with the Chicago Sun-Times, where he disclosed his plans, Mr. Rush said his decision came after a conversation with his 19-year-old grandson, Jonathan, saying that he did not want his grandchildren “to know me from a television news clip or something they read in a newspaper.”

Two dozen House Democrats have now announced their plans to either retire or seek a different political office before the November election, when many Democrats fear they will lose control of the House. With the departure of several senior lawmakers, Democrats face a loss of institutional knowledge and experience.

Although Mr. Rush’s district is heavily Democratic and unlikely to switch parties, the House Republican campaign arm gloated over what Mike Berg, a spokesman, described as evidence that “Democrats are abandoning ship as fast as possible because they know their majority is doomed.”

Mr. Rush is set to hold a news conference at Roberts Temple Church of God in Christ in Chicago on Tuesday. The funeral of Emmett Till, a 14-year-old Black teenager whose murder by two white men helped shape the civil rights movement, was held at the church in 1955.

Before winning the seat in 1992, Mr. Rush enlisted in the Army, helped found the Illinois Black Panther Party and served as a Chicago alderman. As he climbed in Democratic electoral politics, he faced criticism from some other Black activists that he had become too mainstream, but he defeated multiple bids to oust him, including a primary challenge by former President Barack Obama, then a state senator. He was set to face another primary this year.

In the 16-minute video, Mr. Rush reflected on the critical funding he had funneled toward his district and the legislation he had shepherded into law. He has long fought against gun violence and continued his civil rights activism in the House, breaching chamber rules on dress in 2012 when he wore a hooded sweatshirt and sunglasses to honor Trayvon Martin, a Black teenager killed in Florida.

The speech, he said in the video, was a chance to “let the whole world see that although I’m a member of Congress, I’m still a Black man in America who’s fighting for justice and equality.”

He vowed to continue pushing for bills to be passed before his retirement, including the Emmett Till Anti-Lynching Act, which would explicitly make lynching a federal crime.

“I’m going to get this bill passed,” he pledged.