The House passed an Endangered Species Act rewrite last year that horrified many scientists and environmentalists. One environmental lobbyist said the bill amounted to a "death warrant for treasured American wildlife."

They are lobbying the Senate now, in hopes its Environment and Public Works Committee will take a more moderate stance.

"Unfortunately, recent legislative proposals would critically weaken" the law's scientific foundation, said the letter organized by the Union of Concerned Scientists. The 5,738 signers included six National Medal of Science recipients.

"For species conservation to continue, it is imperative both that the scientific principles embodied in the act are maintained, and that the act is strengthened, fully implemented, and adequately funded."

The House bill, written by Resources Committee Chairman Richard Pombo, R-Calif., would require the government to compensate property owners if the steps needed to protect species thwart development plans. It also would stop the government from designating "critical habitat" where development is limited.

Scientists are particularly concerned about how science is handled, including a provision that would give the interior secretary the job of determining what constitutes appropriate scientific data for decision-making.

Pombo said the current law has been a failure, and that his bill would "put the focus on recovery and protect private property owners."

The chairman of the Senate committee, Republican James Inhofe of Oklahoma, is trying to craft an endangered species bill with the committee's top minority member, independent Sen. James Jeffords of Vermont, and leaders of the wildlife subcommittee - Sens. Lincoln Chafee, R-R.I., and Hillary Clinton, D-N.Y.

Inhofe is hoping to have a bipartisan bill later this month, but it's not clear if the senators will be able to reach agreement. Aides said key issues including critical habitat and landowner compensation have not been fully addressed.