"I believe this is the moment when we must stand as one and say enough," Service Employees International Union President Andy Stern wrote in a letter to his fellow members.

While he stopped short of formally opposing the bill that is making its way through the Senate, he did express outrage over the concessions made to Sen. Joseph Lieberman (I-Conn.). "The public option is declared impossible. Americans cannot purchase Medicare at an earlier age. The health insurance reform effort we have needed for a century is at risk," Stern wrote.

And in a significant change of tone, Stern -- who has visited the White House more than any other labor official -- called out President Barack Obama for moving away from the promises of his campaign.

Read more

.