A group dressed in patriotic costumes protest during a "tea party" demonstration in Lafayette Park... 
(Karen Bleier/AFP/Getty Images)
But even if the numbers match elevated expectations, questions are swirling around what's next for the activists.
The movement is leaderless and only aligns indirectly with party politics. While many participants will be Republicans, the anti-spending message is more closely aligned with libertarian themes of small government, with many people angry at both Democrats and Republicans.
"These are folks who have never been involved in the political process before," said Eric Odom, who designed and is running two Web sites to connect supporters and corral information about the protests. Odom said he supported Libertarian Bob Barr for president last year.
"This is a birth of a completely new movement, with a new face, that hasn't been seen anywhere in the country," Odom said.
The question of what to do with the organizing energy will hover over the day's events.
"It's a good and complicated question. When you have a citizen movement, who moves the citizens?" said Roger L. Simon, who has helped spread word of the protests through his Web site Pajamas Media.
Regardless of what happens, the tea parties have made a mark. Fox News Channel has joined conservative bloggers and radio hosts in promoting the events for weeks; Fox is planning several hours of live coverage at different sites around the country today.
At the White House Tuesday, Press Secretary Robert Gibbs said he wasn't sure if Obama was aware of the tea parties but said the president would be using the April 15 tax filing deadline to make a point of his own.
"I think the president will use tomorrow as a day to have an event here at the White House to signal the important steps in the economic recovery and reinvestment plan that cut taxes for 95 percent of working families in America, just as the president proposed doing, cuts in taxes and tax credits for the creation of clean energy jobs," Gibbs said.
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