Unions leaders, among the most passionate backers of President Barack Obama's health care overhaul, pressed Democratic senators Thursday to drop a tax on high-value insurance plans to pay for remaking the nation's system.

As the Senate entered its 11th straight day of debate on the sweeping legislation, members of several labor unions denounced the proposed tax on so-called "Cadillac plans," arguing it wouldn't just hit CEOs but also middle-class Americans who passed on salary increases to negotiate better health benefits.

"I support health care reform but I can't afford this tax," Valerie Castle Stanley, an AT&T call center worker and member of the Communications Workers of America, said at a news conference outside the Capitol. "For families like mine that are on a budget, the results will be devastating."

At issue is a proposed 40 percent excise tax on insurance companies, keyed to premiums paid on health care plans costing more than $8,500 annually for individuals and $23,000 for families. The tax would raise some $150 billion over 10 years to help pay for the Democrats' nearly $1 trillion health care bill. The legislation, which appears to be edging closer to passage, would revamp the U.S. health care system with new requirements on individuals and employers designed to extend health coverage to more than 30 million uninsured Americans.