The White House on Friday claimed it was unaware of the planned rally.
"I don't know who the group is," White House spokesman Robert Gibbs told reporters with a shrug.
But a House leadership aide warned fellow Democrats that up to 2 million demonstrators could turn out.
"It looks like Saturday's event is going to be a huge gathering, estimates ranging from hundreds of thousands to 2 million people," Doug Thornell, an aide to Rep. Chris Van Hollen, D-Md., wrote in a memo obtained by FOXNews.com.
But conservatives believe the memo is ploy to inflate expectations for the turnout anticipating that it will fall short.
"It's an old political tactic to get out in front and make wild projections and when they're not met, claim their opponents don't have the juice," said Pete Sepp, a spokesman for the National Taxpayers Union, one of the organizers of the rally.
The Associated Press contributed to this report.