Sen. Bobby Zirkin, the chairman of the Maryland Senate Judicial Proceedings Committee, plans to resign from the General Assembly as it undergoes a major shift in leadership.

Zirkin (D-Baltimore County) leaves as Senate President Thomas V. Mike Miller Jr. (D-Calvert), a close ally, steps down after three decades leading the chamber. Zirkin said he intends to focus on his law practice, his family and a new project helping professional athletes with charities.

“I’ve been thinking about this for years,” Zirkin, 48, told the Baltimore Sun, which first reported his resignation plans. “I made a commitment to Mike to come back. Now that he’s stepping down, it’s the right time for me.”

Zirkin’s departure from the Senate is the second announced in recent days: Sen. Shirley ­Nathan-Pulliam (D-Baltimore County), who served as vice chair of the Senate Education, Health and Environmental Affairs Committee, resigned over the weekend because of health problems.

The judicial proceedings panel has considered several contentious issues during the five years Zirkin served as chair, including medically assisted suicide and criminal justice and bail reform. He is known as a moderate who often tempered, slowed or blocked the passage of more liberal legislation.

Miller, who is battling cancer and will keep his Senate seat even as he relinquishes his leadership post, called Zirkin a leader on a variety of issues, including juvenile services, cyberbullying and marijuana decriminalization. “He is the definition of a bipartisan public servant, willing to make the hard decisions and lead the state, regardless of special interest pressure,” Miller said in a statement.

Left-leaning lawmakers saw him as the primary obstacle to bills such as the 2017 Trust Act, which would have limited police cooperation with federal immigration-enforcement efforts. After his committee approved a watered-down version of the bill, disappointed advocates held a rally outside the State House, and Del. Joseline A. Peña-Melnyk (D-Prince George’s) called Zirkin a “Democrat in name only.”

He also was criticized for trying to revive Maryland’s bail program after the state’s highest court instructed judges not to set bonds that are too high for poor defendants to pay, and for blocking efforts to change the state’s parole system.

Some liberal Democrats launched an effort to unseat Zirkin in 2018, but he won reelection by a wide margin.

Sen. Bill Ferguson (D-Baltimore City), the incoming Senate president, said in a statement that Zirkin has shown him “not just how to judge policy, but also that the popular decision is not always the correct decision, and that policy hinges on the complexities, not the talking points.”

Ferguson is more liberal — and decades younger — than Miller, and he could move the Senate to the left by appointing someone from the party’s liberal flank to chair the Judicial Proceedings Committee.

“It definitely opens the door to different policy decisions,” said Sen. William C. Smith Jr. (D-Montgomery), vice chair of the committee and a contender to become chair.

Mileah Kromer, a political science professor at Goucher College, said Smith, as chair, would represent a significant shift from Zirkin. She noted that Smith spoke earlier this year at a rally by gun-control advocates, who were critical of Zirkin for not supporting stricter laws.

The House of Delegates is also under new leadership, with Adrienne A. Jones (D-Baltimore County) replacing the late speaker Michael E. Busch (D-Anne Arundel), who died in April.

Zirkin was elected to the Senate in 2007 after eight years in the House. He became chair of the Judicial Proceedings Committee in 2015, succeeding former senator Brian E. Frosh, who was elected Maryland’s attorney general.

Gov. Larry Hogan (R) on Tuesday called Zirkin a “constructive partner in government.” He said in a statement that the two worked together to address violent crime and an initiative to reduce the prison population and costs, and help offenders reenter society.

Sen. Justin D. Ready (R-Carroll), who serves on the Judicial Proceedings Committee, tweeted that Zirkin’s resignation is a “huge loss for the Maryland Senate and the state,” and praised him for championing criminal justice reform and being “very fair” working across the aisle.

The Democratic Central Committee in Baltimore County will recommend a replacement for Zirkin to the governor.

Erin Cox contributed to this report.