WASHINGTON (Reuters) - The White House's top economic adviser, Jason Furman, said on Tuesday that the United States has ample capacity to boost spending to cushion any future economic downturn. Furman, at a news conference on President Barack Obama's final budget proposal, said that gross domestic product growth may be slowing somewhat, but that is being offset in a fiscal context by lower unemployment and lower-than-expected interest rates, and a continued decline in healthcare costs. "Our country has the fiscal room, if and when it is ever needed at some hypothetical future date, to use fiscal policy as one of the tools that we would deploy in those circumstances," Furman said. (Reporting by David Lawder; Editing by Eric Beech)