Showbiz is getting into the health care fight.

The presidents of SAG, the WGA and the Hollywood Teamsters have asked Congress to back off a proposed tax that would hit the “Cadillac” health plans that cover members of their unions. The letter sent to House and Senate reps earlier this month was signed by SAG’s Ken Howard, WGA West’s John Wells, WGA East’s Michael Winship and Teamsters Local 399’s Leo Reed.

The quartet said they were concerned by provisions of the bill passed by the Senate Finance Committee last month that would levy a new tax against high-value health care plans, or so-called “Cadillac plans.” Such a tax could put a serious dent in the finances of union health plans that have already been battered by stock market declines and ever-rising health care costs.

The House narrowly approved a health care reform bill late Saturday after weeks of bare-knuckle partisan brawling over the contentious issue. Democrats in the Senate have said they are hoping to hold a vote on their health care reform package before Christmas. At this point, it’s uncertain if the final version of the Senate bill will include any tax on “Cadillac” plans — but showbiz union leaders aren’t waiting to weigh in.

“We applaud your efforts to expand affordable health care coverage to more Americans and to reform the health insurance industry,” the letter from the guild toppers said. “However, we are deeply troubled by the provisions of the bill passed by the Senate Finance Committee that would levy a new tax against so-called ‘Cadillac plans.’ The individual unions and guilds of the entertainment industry have struggled and sacrificed for decades to negotiate and defend their own Taft-Hartley health insurance plans.”

The presidents said the Senate Finance version of the bill would place “catastrophic” burdens on maintaining health benefits.

“For decades our individual guilds and unions have had to forgo wage increases and other benefits simply to maintain the current level of health benefits for our members,” the letter said. “It is a choice our members were forced to make over and over at the bargaining table – affordable health care versus better wages – and our members chose health care for themselves and their families every time.”

If the proposed tax is enacted, it could “jeopardize our ability to keep our plans solvent,” the letter said.

The unions’ health and pension plans are funded by employer contributions and administered jointly by reps of the unions and the employers. SAG’s pension and health plans, for example, receive a payment from the employer equal to 15% of an actor’s compensation for any given job.

The plans have been hit by the decline in the stock market — which has lowered the value of their investments — and the generally slowdown in showbiz employment, particularly in the highest-paying job classifications of primetime network TV and feature films.