WASHINGTON — With a $33 billion endowment, the Bill & Melinda Gates Foundation is the world’s largest philanthropy. If you add in donations expected from billionaire Warren Buffett, the charity’s assets will grow to twice that size, making them nearly as large as those of oil-rich Kuwait.

But apparently that’s not big enough. The foundation also is going after money from American taxpayers. This month, it gave a three-year, $3.8 million grant to Families USA, a liberal advocacy group, to coax billions of dollars from the American government to advance the cause of global health.

For 25 years, Families USA has been one of Washington’s loudest and most effective cheerleaders for expanding the government’s role in health services in America. It’s currently beating the drum for legislation that would provide health coverage for the uninsured and cut the cost of prescription drugs under Medicare.

But now, thanks to the Gates Foundation grant, Families USA will also shake the capital’s money tree for health services around the world. Joe Cerrell, the foundation’s director of global health advocacy, said he turned to Families USA to “make the case why government should step up and fund more generously programs that have a direct impact on saving millions of lives.”

The idea of leveraging the foundation’s largess to scare up more money from elsewhere is relatively new — four or five years old, Mr. Cerrell said. Since 1994, the Gates Foundation and its predecessors have doled out more than $13 billion, much of it to directly purchase vaccines and other medicines to battle diseases that proliferate in impoverished countries. But to meet its goal of eradicating the world’s worst scourges, Mr. Cerrell said, it needs a multiple of that amount.

So it is looking for assistance from entities with even deeper pockets, including Uncle Sam. “The foundation’s resources alone aren’t enough to fill that gap,” Mr. Cerrell said.

Families USA said it would use a variety of tactics familiar to anyone on K Street to get the attention of official Washington. “We’ll be putting out research, developing messages, and doing a lot of media and communications outreach in Washington and other parts of the country,” said Ron Pollack, executive director of Families USA. “Our hope is to convince the public, opinion leaders and policymakers that this is a good thing to do.”

Specifically, Mr. Pollack said he wants to find ways “to increase and improve funding for research on malaria, tuberculosis, HIV/AIDS, and other tropical diseases prevalent in developing countries.”