Sen. Barbara Boxer of California is privately lobbying fellow Democrats on the Environment and Public Works Committee against a toxic chemical bill negotiated by Sen. Frank R. Lautenberg in the weeks before his death.

The bipartisan legislation is viewed by many close to Lautenberg as a legacy piece for the late New Jersey Democrat. It would revamp nearly 40-year-old chemical laws, a goal Lautenberg pursued for years before he died June 3 at the age of 89.

But internal division over whether the bill is strong enough led committee Democrats to schedule a Thursday meeting to discuss the measure that Lautenberg negotiated with Republican David Vitter of Louisiana and Democrat Joe Manchin III of West Virginia.

That trio announced at the end of May it had reached a compromise on the measure, and three of the top four Senate Democratic leaders have signed on as co-sponsors. But the fate of the bill is now largely in the hands of Boxer, the chairwoman of the committee of jurisdiction.

The measure would update the original law from 1976 and would institute comprehensive chemical safety regulations by requiring all chemicals to be screened for safety and labeled according to their potential risk to human health or the environment. Among other provisions, the legislation also would authorize the Environmental Protection Agency to take action against a chemical found to be unsafe, either through tougher labeling or bans on that chemical.

One particular issue of concern to Boxer, whose home state of California has rigorous toxic chemical laws, is that the weaker Lautenberg-Vitter bill would pre-empt local laws.

Beyond the merits of the bill, however, Boxer and her staff also have questioned Lautenberg’s capacity to agree to the legislation in the final days of his life, according to multiple sources, including senators.

According to these sources, Boxer has been engaged in senator-to-senator conversations and has argued that Lautenberg either didn’t sign off on the final language or wasn’t in a position to, given the illness that ultimately took his life. Senate Majority Whip Richard J. Durbin, D-Ill., is one of 20 co-sponsors of the Lautenberg-Vitter bill and one of the members whom Boxer spoke to directly.

“She raised it to me, and I’m glad she did because I assumed Frank signed on, but I understand the circumstances when it happened.” Durbin said in an interview. “I looked at [the bill] again, and there are two parts of it that trouble me. The first part is the pre-emption of state standards. The second part is the elimination of private causes of action. So I’m going to stay with the bill, but before I would vote for it, I’d have to see those two [issues] addressed.”

Boxer, in a phone interview with CQ Roll Call, acknowledged having conversations with senators about the bill but said she never argued that Lautenberg hadn’t actually signed off on it nor that he wasn’t capable of doing so.

“I have no idea [if Lautenberg signed off] because I have no idea about how this bill was written because I wasn’t involved and only knew about it when I read about it in the papers,” Boxer said.

“David Vitter could tell you how involved Frank Lautenberg was,” she said. “I would never say that Frank signed off on it or not signed off on it because I was not involved so I wouldn’t know.”

Multiple sources close to the negotiations say Lautenberg was engaged throughout the process.

These sources also maintain that, at various points since Lautenberg’s death, Boxer and her committee staff director, Bettina Poirier, have approached other senators and aides about Lautenberg’s state of mind when he signed off on the bill. Poirier has been more active than her boss in campaigning against the Lautenberg-Vitter compromise, according to people who are familiar with the lobbying.

Manchin, who helped bridge the gaps between Lautenberg and Vitter, told CQ Roll Call that Lautenberg was engaged in the process.

“He was involved, and I saw him the last time he was on the floor and thanked him after the agreement had been reached,” Manchin said. “I’m hoping that [Boxer] is going to be able to look at all the work that’s been done. Every senator dedicates an awful lot of their energy toward certain, specific projects, and this is one Frank has been on for many years. So he had a great deal of knowledge, his staff had a great deal of knowledge. They worked on it.”

A picture posted to Lautenberg’s Facebook page on the day of bill’s announcement shows him and Vitter, the top Republican on the committee, on a couch talking, Lautenberg with a document in hand.

Aides to Lautenberg declined comment for this story. But advocates of improved chemical safety laws have argued the issue would be one of his most lasting legacies. Frances Beinecke, president of the Natural Resources Defense Council, told Mother Jones after Lautenberg died that, “Perhaps his most enduring achievement was to help inform and protect the public from the harm of toxic chemicals, including creating the nation’s toxic right-to-know law, establishing the U.S. Chemical Safety Board and pushing for greater security at chemical plants.”

Boxer said she told Lautenberg’s staff that the committee would work on a chairman’s mark, but sources close to the panel said her staff has essentially shut out Lautenberg’s legislative team. Aides to Lautenberg have 60 days on the general Senate payroll after his death before they are no longer employees of the Senate.

“I think we will work off that bill, but I can’t tell you for sure,” Boxer said when asked if the Lautenberg-Vitter measure would serve as the base text moving forward.

Manchin said Tuesday he already has reached out to Boxer about the bill. “I called Sen. Boxer and asked her, and she said she wants to work it through her committee and that everyone would have input on this,” he said. “I would hope that they would honor Frank Lautenberg’s lifetime of dedication.”