Accompanied by several first responders, Stewart met with members of Congress and attended a Democratic caucus luncheon.
"If you can't get this done, maybe we should shut down," Stewart told reporters outside the U.S. Senate, referring to the possibility of a government shutdown this fall. "This is about as unassailable a piece of legislation you can have."
Stewart was host of Comedy Central's "The Daily Show" for 16 years before stepping down in August.
The law, which is set to expire next month, established the World Trade Center Health Program to provide medical monitoring and treatment for first responders affected by Sept. 11-related illnesses such as pulmonary diseases and cancers. The Zadroga Act, named after a responder who died after working at ground zero, first became law in 2010 after a debate over the bill's cost.
Proponents are seeking the law's permanent extension in part because some illnesses may not manifest until years later, after the statute of limitations for worker's compensation or certain state laws may have run out. House Republicans are supportive of the program but have opposed its permanent extension because they say they want the chance to periodically review it and make sure it is operating soundly.
Earlier this summer, Dr. John Howard, the administrator of the World Trade Center Health Program, told members of the House Energy and Commerce Committee that extending the law would help clinicians treat victims, and allow administrators to better plan patient care.
"It's stressful to be told on a year-to-year basis that your care might be taken away," Howard said. "From the administrative perspective, it's stressful because we have to constantly prepare for when this may end."
Sen. Kristen Gillibrand, D-N.Y., accompanied Stewart Wednesday as he lobbied for the bill.
"It's a moral obligation and I think people do understand that," she said. "I am very optimistic that we can bring the entire Senate together to support this bill."
As Stewart talked to members, Senate Majority Leader Mitch McConnell, R-Ky., said Congress would extend the program, but did not provide details or say whether it would be done permanently.
New York's other senator, Democrat Charles Schumer, said McConnell's words are "a glimmer of hope."
"If Congress can't come together and help the first responders who are ill because they rushed to the towers, then we may as well forget this place all together," Schumer said.