A hearing to debate the nation’s decades-old toxic chemical laws turned into a power struggle Wednesday, as Environment and Public Works Chairwoman Barbara Boxer, D-Calif., tried to hold her ground against a growing coalition that backs a bipartisan compromise to which she was not a party.

The extensive hearing, packed with 19 listed witnesses, was at times personal and tense, with the shadow of the late Sen. Frank R. Lautenberg, D-N.J., cast over the proceedings. Members debated the compromise legislation Lautenberg negotiated in the final weeks of his life with the panel’s ranking member, Sen. David Vitter, R-La. — a bill against which Boxer has campaigned vigorously and is opposed by some environmental groups looking for a stronger regulation.

Perhaps the most personal testimony came from the hearing’s one unlisted witness, Sen. Joe Manchin III, D-WVa., who defended Lautenberg from criticism launched by Boxer that the late senator was unfit to agree to a deal, as CQ Roll Call previously reported. (Here, here and here.)

“I was disheartened when I read news reports that some members of this committee questioned Senator Lautenberg’s faculties during the CSIA negotiations,” Manchin said. “He entered into these negotiations, with all his years of experience, skills and wisdom because he knew that it was time to craft a bipartisan TSCA reform bill. To suggest otherwise, and to attack the integrity of such a strong defender of public health, is something that should offend all Americans.”

Manchin said Lautenberg “negotiated the agreement before you because he knew it was time to fix this broken system in a way that could pass Congress and make Americans safer” and added that any claim that the deal, which Manchin helped broker, was “worse” than current law was “just another cheap attempt to try and distort the tireless work Senator Lautenberg gave to the bill.”

Democrat Tom Udall, D-N.M., the chairman of the toxic chemicals subcommittee and supporter of the Vitter-Lautenberg bill, read a letter from Lautenberg’s widow, Bonnie Englebardt Lautenberg, who said passing the legislation would be a “wonderful cap” to the late senator’s career.

The hearing marked an inflection point for Boxer, whom sources say had hoped to use the session to her favor and in support of moving her own legislation. But several Democrats, chiefly Udall, seemed aligned with Vitter’s position. These sources suggested that EPW staffers had been considering moving forward with their own bill over recess, but doing so could cause Boxer to lose all control of the issue.

That tension came to a head during a particularly awkward exchange between Vitter and Boxer, as Vitter announced to the panel he would be working on a “manager’s amendment” with Udall. Given that chairmen usually are the managers of bills coming out of committee, such a declaration was either a serious faux pas or a direct shot across the bow at Boxer, whose staff to date has not been willing to look at changes to the existing language.

Vitter was explaining to the committee that he and Lautenberg did not intend to undercut state laws, such as California’s, when he said he and Udall would make that “crystal clear in our manager’s amendment.”

At this point, Boxer interrupted and said, “I just wanted to ask you, what do you mean, ‘manager’s amendment?’ Because I will be managing the bill. I just wanted to make it clear that I would be working the manager’s amendment with you, unless you don’t like it and I can get somebody else who does.”

Senators moving behind the scenes to undermine a committee chairman is one thing, but practically declaring to her face that you already are, is quite another, and Boxer’s curt response underscores her tenuous position.

She could decide to move ahead without this coalition, but she would risk alienating her own members and not passing any bill at all. Though sources working close to the bill are loath to say so publicly, they could move forward outside of the committee construct and ask Senate Majority Leader Harry Reid, D-Nev., for a floor vote once they’re done amending the bill. The legislation already has about 20 co-sponsors from both parties.

“It’s just time that something be done. I would hope that the chairman and her committee would say this is the time and it would happen. Frank reached out, finally came to a compromise,” Manchin told CQ Roll Call after his testimony.

When asked if allies of the bill would work around the EPW panel if they faced resistance, Manchin would only say that Reid “will have to make a decision where he goes from there.” He added that he had not yet talked the majority leader but “will at the proper time.”