Thimerosal is an additive the company once used in its vaccines, and lawsuits allege that the additive contributed to autism and other neurological problems in children, although there is little or no medical evidence for such a link.

Exactly who inserted language into the bill remains a mystery. The bill requires that those with complaints about vaccine additives first pursue remedies through the federal Vaccine Injury Compensation Program.

But parents whose lawsuits were canceled by the language have several lawmakers now doing their bidding. "This is the worst kind of special interest legislation," Sen. Patrick Leahy (D-VT) told a news conference outside the Capitol attended by dozens of parents of stricken children.

Leahy on Tuesday joined Sen. Debbie Stabenow (D-MI) in introducing legislation that would repeal the provision enacted in November, which Stabenow said "takes away the legal rights of parents to protect their children." Reps. Tom Allen (D-ME) and Dennis Kucinich (D-OH) are introducing a House version of the bill.

Meanwhile, a second group of lawmakers is working on another approach, which would modify but not repeal the language in question. Just before the Senate passed the bill, Maine Republican Senators Olympia Snowe and Susan Collins, along with Lincoln Chafee (R-RI), threatened to block the bill until House and Senate GOP leaders promised them the language would be revisited in January.

A spokesman for Snowe said the senators have been working with Sen. Judd Gregg (R-NH), the incoming chairman of the Senate Health, Education, Labor and Pensions Committee, on language that would clarify parents' ability to use the federal vaccine compensation program for thimerosal claims, even if their children's injuries occurred outside the normal three-year time limit.

Complicating matters is the fact that the pledge to revisit the matter in the Senate came from Trent Lott (R-MS), who has since been replaced as Republican leader by Bill Frist (R-TN). Frist wrote the language that was inserted in the homeland security bill, originally part of a broader vaccine measure, and while he has denied seeking its inclusion in the bill, he defended it on the Senate floor during the homeland security debate.

Nonetheless, Frist has agreed to keep Lott's promise to reconsider the matter, a spokesman said. "We are continuing to talk with them to address their concerns," he said of the Snowe-Collins-Chafee-Gregg group.