News that Sen. Tom Harkin, D-Iowa, plans to retire next year is leaving disability advocates pondering a future without their staunchest ally on Capitol Hill.

Harkin, 73, said Saturday that he will not seek a sixth term in the U.S. Senate next year.

“When the current Congress is over, I will have served in the United States House of Representatives and the U.S. Senate for a total of 40 years. After 40 years, I just feel it’s somebody else’s turn,” Harkin said in a statement.

The departure will leave a gaping hole in Congress for the disability community, advocates say. Harkin was the leading force behind the landmark Americans with Disabilities Act and has played an active role in countless other legislative efforts promoting everything from education and employment to health care and independent living opportunities for those with special needs.

“Sen. Harkin has been a huge champion in the disability community,” said Mark Perriello, president and CEO of the American Association of People with Disabilities. “Moving forward, I hope that we can find someone who can step into his very large shoes and it’s going to be difficult.”

Though Perriello said there are many lawmakers on Capitol Hill who are supportive of the disability community, he was hard-pressed to name another legislator who might step up the way Harkin has. Last year, for example, when opposition emerged over new accessibility requirements for swimming pools, it was Harkin who went to bat for people with disabilities.

“He was the only one paying attention,” Perriello said. “He was the only one willing to stand up whether the issues were big or small.”

Harkin speaks often of his late brother, Frank, who was deaf and whose experiences largely inspired the lawmaker’s efforts to make life better for Americans with disabilities.

While many members of Congress are sympathetic to the needs of those with disabilities, Harkin has a “laser focus” on this population that’s not currently shared by any of his colleagues, said Katy Neas, who worked for Harkin as a congressional staffer in the 1980s and early 1990s and currently handles government relations for Easter Seals.

“Every piece of legislation that comes before him he asks, ‘what does this mean for people with disabilities?’ His dedication to our population is really second to none,” Neas said.

Without such an ally in the Senate, Neas said disability advocates will soon be left to persuade lawmakers who have other items at the top of their agendas.

“We’re going to have our work cut out for us educating legislators about why people with disabilities should be a priority,” she said.