Washington ? Labor unions are joining employer groups in support of a new rule that would let companies reduce or eliminate health benefits for retirees when they become eligible for Medicare.

The Equal Employment Opportunity Commission voted 3-1 along party lines Thursday to approve the rule, which is subject to further review before it becomes final.

It’s a politically potent issue this year that is stirring a large and powerful voting bloc and is boxing in lawmakers friendly to labor.

Capitol Hill offices were flooded with phone calls from retirees objecting to the regulation, which is subject to further review before it becomes final. AARP has strongly opposed it.

Democrats tried to shift focus to the Bush administration. EEOC’s decision “highlights the failure of the Bush administration to bring down the high cost of health care and improve the quality of health care for all Americans,” said Sen. Edward Kennedy, top Democrat on the Senate Health, Education, Labor and Pensions Committee.

Aides are studying the regulation’s potential impact, said Kennedy spokesman Jim Manley.

The public reaction to the rule prompted the EEOC to release a statement Friday to retirees. “We know that retiree health benefits are very important to you,” said Chairwoman Cari M. Dominguez. “We want to reassure you that (EEOC) is acting to safeguard these benefits, not to eliminate them.”

Democrat John Kerry’s campaign said even more retirees are at risk of losing benefits when the prescription drug bill signed by President Bush last year takes effect next year.

The Congressional Budget Office estimated that 2.7 million retirees will lose the drug coverage they now receive from former employers, although other projections are that a much smaller number will have their existing private benefits dropped.