Both sides of the health care debate are relying on a grassroots army.

As Congress enters its second year of debate on an overhaul of the nation's health care system, two major national groups are continuing to reach out to millions of grassroots advocates.

The Service Employees International Union, which represents health care and public service workers, keeps in touch with supporters of the overhaul. The U.S. Chamber of Commerce, which represents business interests, is contacting opponents.

Despite their different thoughts on the issue, the two groups have a similar approach.

William C. Miller Jr., who runs grassroots efforts for the Chamber, regularly updates its base of 3 million supporters, which he calls "the family." They include employees of trade associations, local chambers of commerce and other pro-business supporters.

He doesn't contact each of them directly, instead sharing his team's analysis through a hierarchy of regional officers and leaders, who in turn inform local boards and members.

"The family becomes stronger when our extended cousins are constantly getting news about what's happening with the family in D.C.," Miller said.

The grassroots has really become an important part of how the Chamber operates in recent years. In 2009, the Chamber's members contacted their lawmakers 1.19 million times, up from 50,000 communications the year prior.

Members of Congress now know that an endorsement from the Chamber can mean thousands of votes, Miller noted.

But the member-Chamber relationship is a two-way street, Miller added. The 96 percent of its members who are small businesses need "the guys in Washington who are fighting on [their] behalf."