After years of strenuous opposition, Wal-Mart, the nation's largest private employer, announced that it supports a controversial proposal requiring businesses to contribute to the cost of employee health insurance. Wal-Mart's endorsement comes as the health reform debate intensifies, and it could have broad economic and political consequences. Many business groups, displeased with the shape of the legislation that has emerged so far, have begun to mobilize against President Obama's reform efforts.