Those who believe that childhood vaccines (search) may be linked with the nation’s soaring rates of autism in children are backing efforts on Capitol Hill to remove all mercury from inoculations, but are also fighting legislation they see has shielding drug makers from future litigation.

"There has been a concerted effort by some Republicans who seem to be more concerned about protecting big business than protecting children," said Lujene Clark, who with her husband Dr. Alan Clark, runs NoMercury, a Missouri-based grassroots organization committed to disseminating information about the mercury-containing vaccine preservative, thimerosal (search).

The Clarks' son began showing signs of neurological damage after he got a flu shot three years ago when he was 7. She and her husband say this was his "toxic tipping point" after years of high-mercury exposures.

"Our whole purpose is to convince policy-makers that it is prudent to use cautionary principles," she said, "not to expose children to [mercury], not to expose pregnant women to it. Personally, we don't want any person exposed to it."

Not everyone agrees with the Clarks' assessment.

While it is shared by a growing number of parents' groups, government watchdogs and researchers, the medical and scientific establishments remain firm that a link between childhood vaccines and autism (search) has not yet been found.

Editor's Note: This is the second in two parts exploring the debate over the possible link between childhood vaccines and autism. This installment looks at the politics of the controversial issue. Click here for the first part, which examined both sides of the medical and scientific debate.

According to the U.S. Department of Education, the number of children age 6 through 21 with autism has increased 500 percent over the last decade, reaching over 140,000 in 2004. Experts estimate that one in 166 children in the U.S. is diagnosed today with some form of autism.

At the same time, the number of vaccines children get in their early years has increased from about a handful in the 1950s to an estimated 22 shots today.

A Decade-Long Fight

This battle has long been fought on Capitol Hill — from the establishment of the National Vaccine Injury Compensation Fund 10 years ago in response to parents' concerns, to current debates over the existence of mercury in all vaccines.

But a renewed vigor today among parent groups and a swath of researchers who claim that vaccines are indeed the cause of increased autism among children has some lawmakers on the defense, while others see this as an opportunity to get tough legislative action on vaccines finally passed.

"What is troublesome is the polarization," said Dr. Lewis Cooper (search), professor emeritus of pediatric medicine at Columbia University, who has been working with vaccines for 40 years, and says the rhetoric today over what is considered safe or dangerous has been heated and not always accurate.

"In my view," said Cooper, who was president of the American Academy of Pediatrics (search) when it released the statement with the U.S. Public Health Service in 1999 saying thimerosal in vaccines should be removed voluntary, "the evidence favors rejecting any causal relationship between thimerosal and vaccines and autism."

His view is bolstered by the Centers for Disease Control and Prevention (search) as well as the American Academy of Pediatrics, which said in 2004 that there is no credible link between thimerosal in vaccines and the hike in autism rates.

But in response to studies on mercury in vaccines and its affects on children prior to 2004, the federal government urged drug makers to rid thimerosal from all childhood vaccines in 1999. It also said more than one microgram of mercury in a single vaccine dose was unsafe for children. Up until that time, single-dose vaccines with thimerosal carried as much as 25 micrograms.

As a result, drugmakers made all childhood vaccines thimerosal-free by 2002, but critics contend that some still have traces. Adolescent and adult vaccines, as well as some child flu vaccines, still carry the thimerosal preservative, too.

Rep. Dave Weldon (search), R-Fla., who is a physician, has been fighting mercury in vaccines for years and wants it out once and for all.

"In my opinion, mercury should not be put in any biological product — I think the substance needs to be banned," said Weldon, who dismisses the 2004 report as inconclusive "garbage."

He and Rep. Carolyn Maloney, D-N.Y., have re-introduced the Mercury-Free Vaccines Act of 2005 (search), which would officially ban thimerosal from all childhood and adult vaccines by 2009.

The legislation has 35 bipartisan co-sponsors, and now awaits action in the Health Subcommittee of the House Energy and Commerce Committee.

"Mercury is a toxic substance that causes serious developmental problems in children and infants," charges Rep. Michael Michaud, D-Maine, a co-sponsor. "We should make every effort to keep young people safe."

Finding lawmakers who publicly disagree is difficult, but privately, officials say all of the available research must be balanced before changing the law. Cooper cautions against letting rhetoric affect the perception of childhood vaccines, which save millions of lives each year.

"I think [Weldon's bill] is premature," he said, noting that thimerosal is a necessary preservative, particularly for multi-dose vaccine vials used for disbursing mass doses of vaccines during flu season.

Another Bill, Another Approach

Parent groups say the Protecting America in the War on Terror Act of 2005, otherwise known as Senate Bill 3, contains language that would shield childhood vaccine makers from future lawsuits by capping damages and throwing all such suits into federal courts.

They also contend that the bill would preclude state laws today banning mercury from childhood vaccines. California and Iowa have such bans, and bills are currently pending in several states.

Sen. Judd Gregg (search), R-N.H., the main sponsor of the bill, which was introduced in January and referred to the Finance Committee, could not be reached for comment.

Critics have recently attacked co-sponsors Senate Majority Leader Bill Frist, R-Tenn., and Sen. Rick Santorum, R-Pa., on the issue. Both offices declined comment — although Santorum's office pointed out that the senator just introduced legislation in April boosting autism research to $860 million over the next five years.

Senate sources who did not want to be identified said critics were misreading the bill, the provisions of which address vaccines for endemics, pandemics and bio-terrorism attacks — not childhood vaccines.

Sources said that the bill seeks to encourage vaccine makers to stockpile vaccines and treatments necessary for catastrophic events. They said it did not apply to the use of childhood vaccines at all.

Secondly, they said, there is no provision that would preclude state bans on thimerosal in vaccines, and discussions have revolved around clarifying the language addressing that.

Clark doesn't buy it. "We're not stupid, we do read English," she charged. "It's worded loosely enough that a strong case can be made that it could include the thimerosal [issue]."

Steven Milloy, author of "Junk Science Judo: Self-defense Against Health Scares and Scams" and a FOXNews.com contributor, says conspiracy theorists are trying to drive policy, and this is a classic case.

"It's scary," he said, "because there is no science on their side."