After a week of anger and criticism from veterans groups and advocates, including the comedian Jon Stewart, Senate Republicans finally agreed to pass a bill that expands healthcare and benefits to veterans exposed to toxins.

“I’m not sure I’ve ever seen this situation where people who have already given so much had to fight so hard to get so little,” said Stewart, who joined veterans to push for the bill. “I hope we learned a lesson.”

Known as the Pact Act, the bill passed in the Senate on Tuesday night in an 86-11 vote.

It will provide assistance to veterans who were exposed to harmful chemicals during their service, such as Agent Orange during the Vietnam war, or toxins from pits used to burn military waste in Iraq and Afghanistan.

The Department of Defense estimates that about 3.5 million service members could have been exposed to burn pits in the Middle East. Such chemicals can cause respiratory illnesses and cancer to those exposed, medical experts said.

Currently veterans have had to prove that illnesses were connected to their service, and the Department of Veterans Affairs did not consider exposure to toxins a service-related condition. The department has denied about 75% of veterans’ burn pit claims.

The Pact Act passed the House last month and had support from a majority of Republican senators until last week, when a coalition of them held up the bill’s passage. The senators said they no longer supported the bill after Democrats announced they reached a deal on a major tax and climate bill on Wednesday.

Veteran advocates sharply denounced the Republicans’ U-turn. Stewart called them “stab-vets-in-the-back senators”. Protests erupted outside the Capitol.

On Tuesday night, the Senate majority leader, Chuck Schumer, a Democrat, announced that he and his Republican counterpart, the minority leader, Mitch McConnell, had reached a deal on the bill. At a news conference, McConnell said Republicans’ objections were part of the “legislative process”.

“These kind of back and forths happen all the time in the legislative process, you’ve observed that over the years,” he said. “I think in the end, the veterans service organizations will be pleased with the final result.”

Joe Biden praised the bill’s passage, saying: “We should all take pride in this moment.” In his most recent State of the Union address earlier this year, the US president mentioned that his son Beau, who served a year-long tour of Iraq and later died of brain cancer, could have been a victim of burn pit toxins.

“The Pact Act will be the biggest expansion of [veterans affairs] healthcare in decades,” Biden tweeted. “We’ll never be able to repay the debt we owe to those who have worn the uniform, but today, Congress delivered on a promise to our veterans and their families.”