At the urging of some of their biggest corporate constituents, lawmakers are finally entering the fray over a proposal to save about $1.2 billion a year by changing the current Medicare coverage formula for dialysis treatment.

Democratic Sen. Debbie Stabenow of Michigan is the first to publicly come down on the side of employers who would have to pay more for coverage if the changes are made. But ­congressional aides say more lawmakers are expected to voice their concerns — especially if some of their biggest constituents continue to weigh in on the issue.

“Every major employer and manufacturer that provides good health insurance is concerned about this,” Stabenow said Monday.

One of her state’s — and the nation’s — biggest automakers, General Motors Corp., estimates that the change would cost it $70 million a year.

The proposal would lengthen to four years (from the current three years) the waiting period before Medicare steps in to assume the costs of dialysis treatment for kidney patients from their private insurers.

Adding a year would significantly reduce Medicare costs. But companies say it could cost them up to $4 billion a year more in health care insurance costs because the change would lead to higher premiums.

The proposal could be part of an end-of-the-year Senate Finance Committee bill. Stabenow, who is a member of the panel, says she’s urged Chairman Max Baucus, a Democrat from Montana, to keep it out.

The proposal first appeared as a cost saver in a House Medicare bill (HR 3162) aimed at stopping scheduled cuts in Medicare payments to physicians. Lawmakers are exhaustively searching for savings in order to avoid physician cuts.