A demonstration against government spending and “socialized health care” planned for Sept. 12 in Washington could become “the largest gathering of fiscal conservatives ever,” organizers said on Thursday. Among the sponsors of the march is FreedomWorks, a conservative advocacy group that was instrumental in fueling opposition to a health care overhaul at town hall-style meetings this summer. The National Taxpayers Union and the Tea Party Patriots are also major sponsors. The groups began building momentum earlier this year with opposition to President Obama’s $787 billion stimulus plan and bank bailouts. Over the summer, their opposition to big government spending found an outlet at the health-care town halls. Matt Kibbe, president of FreedomWorks, said that the groups had started planning the protest and obtaining permits last February, realizing that Congress would be returning to Washington this month and taking up major legislation, including health care. “Health care has always been one of our primary focuses, and we guessed right about the timing,” he said in an interview. “It is emblematic of this broader theme about spending money we don’t have and the government getting involved in things it has had no success on.” Asked if Medicare was not a success, Mr. Kibbe said it was underfunded, broken and financially unsustainable. Organizers said in a conference call that they expected tens of thousands of people at the march but could not predict a number because many demonstrators planned to come on their own. They also said that the crowds were not being “ginned up.” “All we’ve done is create the space for these guys to show up,” Mr. Kibbe said. “If they aren’t angry enough to pay their own way, we couldn’t gin it up.” Andrew Moylan of National Taxpayers Union said that his group had been around for 40 years and only now were thousands of new people responding to the movement. “The story of this is that it’s not just conservatives anymore, who were always concerned about this,” he said. “It’s the story of the disaffected moderates who have had it up to here with the actions in Washington.” Adam Brandon, a spokesman for FreedomWorks, predicted this would be the largest gathering ever of fiscal conservatives, distinct from the social conservatives who have organized major events around issues like abortion and gun rights. Fiscal conservatives, he said, had never really gathered in groups of more than a couple of thousand. “This is the beginning of a big tent on our side,” he said.