WASHINGTON BUREAU — Wal-Mart Stores Inc., the largest private U.S. employer, says it supports the concept of requiring private employers to offer health coverage.

Mike Duke, chairman of Wal-Mart, Bentonville, Ark., has joined with Andrew Stern, president of the Service Employees International Union, Washington, and John Podesta, chief of the Obama transition and head of the Center for American Progress, Washington, in signing a letter addressed to President Obama that expresses support for an “employer mandate.”

“We are for shared responsibility,” Duke and the others write in the letter. “Not every business can make the same contribution, but everyone must make some contribution.”

Duke and the others note that they also want to see Congress offer a guarantee to businesses that health care costs will be contained.

Wal-Mart, SEIU and the Center for American Progress are calling for use of a trigger mechanism that would impose automatic reductions if health care spending rose above annual targets.

“The promise of savings in the bill must be more than just words,” Leslie Dach, a Wal-Mart representative, says in a statement about the company’s views on health reform. “Wal-Mart believes that if we support a mandate and are being asked to pay higher taxes, we should be assured in return, that savings will be real.”

But Wal-Mart believes the time for comprehensive reform is now, Dach says.

“The present system is not sustainable,” Dach says. “The status quo is not an option.”

The letter was sent as Obama set about trying to round up support for health care and health finance efforts. He traveled to Annandale, Va., today to participate in an emotional public forum on health finance issues.

Wal-Mart is arguing that the amount an employer pays for health coverage should be based not on the number of employees, but on the amount of profit per employee.

Employer mandate opponents include the National Retail Federation, Washington, and the U.S. Chamber of Commerce, Washington.

The kind of profit-based health coverage formula that Wal-Mart is suggesting would favor Wal-Mart and other employers that have many low-wage employees, mandate opponents are saying.

John Greene, a vice president at the National Association of Health Insurers, Arlington, Va., also is voicing concerns about Wal-Mart’s decision to sign the employer responsibility letter.

Greene says he had hoped that Wal-Mart would remain neutral about the coverage mandate issue.

Supporters of the health coverage proposals developed by congressional Democrats may argue that, now that the nation’s biggest employer has agreed to do support the “employer responsibility” concept, others should follow suit, Greene says.

“It gives supporters of the legislation being pushed by Democrats in Congress additional reason to push for an employer mandate in final legislation,” Greene says.

NAHU and most employers oppose a coverage mandate because “it is a job-killer, it depresses wages and, even if such a mandate were to start with a low floor, over time it would increase,” Greene says.