The nation's largest employer thinks big companies should be required to provide health insurance for their workers.

Wal-Mart, the country's biggest retailer and the second-largest corporation in the world according to this year's Fortune 500, this week endorsed the idea of a so-called employer mandate.

That's a key — and controversial — part of President Barack Obama's health reform plan. It's also a feature of the bipartisan Massachusetts health reforms that cut in half the number of uninsured in that state.

Under such a mandate, all but the smallest businesses would be required to make a "fair and reasonable" contribution toward workers' insurance premiums.

Wal-Mart CEO Michael Duke sent a letter to congressional leaders and the White House this week in which he expressed support for the mandate. It was co-signed by Andrew Stern, president of the Service Employees International Union, Wal-Mart's unlikely health care ally in recent years.

Wal-Mart didn't always have such a progressive approach to health care.

In 2005, about a quarter of Wal-Mart workers were either uninsured or on state Medicaid programs with the costs of their care absorbed by taxpayers.

Half the children of Wal-Mart workers were in the same boat. In part, that's because the company offered workers information on applying for Medicaid instead of offering them health insurance.

Wal-Mart workers who were insured were paying about twice the national average in out-of-pocket costs. Four-in-10 were paying as much as 16 percent of their wages.

That changed in part because those shameful facts were outlined in a letter written by a corporate vice president to Wal-Mart's Board of Directors, which was leaked to reporters.

Stung by the bad publicity and by anti-Wal-Mart campaigns funded by unions trying to organize workers, the retail giant changed its ways. Today, more than half of its workers have health insurance provided by their employer, compared with about 40 percent of retail workers nationwide.

Wal-Mart's support comes as Obama's health reforms are under attack from Republicans. The GOP pays lip service to the need for change, but they have no real plan to fix the system.

Duke indicated that Wal-Mart's support is conditioned on how well reforms actually control health costs. "There has to be some sense that the promise of the bill to reduce health costs will actually occur," one of the company's lobbyists told The New York Times.

Specifics of the national mandate, or even whether it will be enacted, remain to be seen. In Massachusetts, all companies with 11 or more workers must contribute to employee health insurance premiums. They must also offer a plan that employees can pay for with pre-tax dollars.

Companies that fail to comply face a fine of up to $295 per worker per year. That's far below the $4,700 national average cost of individual coverage in 2008. That makes it cheaper for some companies simply to pay the fine than provide health benefits.

Adults in Massachusetts also are required to get coverage. Those who don't lose a substantial income tax deduction.

Even with Wal-Mart's support, health reform is far from a done deal. But Duke, the giant retailer's president, got it exactly right in his letter to the White House.

"Not every business can make the same contribution," he wrote, "but everyone must make some contribution."

REPRINTED FROM THE ST. LOUIS POST-DISPATCH.