On the afternoon before Jon Stewart and Stephen Colbert held their joint rally, devoted followers of the two Comedy Central leaders made an early pilgrimage to the stage under the Capitol dome. No one attending knows whether the comedians will restore sanity or preserve fear, but the event is likely to score dynamite television ratings and draw throngs to the Mall.

"I'm really excited to be part of something," said Josie Thompson, 30, who had traveled from Durham, N.C., and posed for pictures with her baby Olive in her arms. "It's going to be huge."

Aaron Thompson, a 37-year-old doctoral student, echoed his wife's excitement for the rally, though he considered that "something" to be an open-air comedy show.

"You don't think that once it's so big, it becomes a movement?" asked his wife. She argued that discussing politics -- even cracking wise about it -- to a sea of worshipful followers amounted to a rather "good pulpit."

Two months after Glenn Beck staged a religiously themed "Restoring Honor" rally on the anniversary of Martin Luther King Jr.'s "I Have a Dream" speech with Sarah Palin on the steps of the Lincoln Memorial, Stewart and Colbert will hold their rally at the opposite end of the Mall. It will also offer the opposite point of view.

The "Rally to Restore Sanity and/or Fear" is billed as a correction to what Stewart, Colbert and their legion fans consider the extremism, vacuousness and dishonesty that has infected the American political system, especially the self-righteousness and conservative fear-mongering they see in Beck and his employer, Fox News. It will include musical interludes by reliable openers such as the Roots and professional rally balladeer Sheryl Crow. It will be emphatically covered by the rating-points and Web-traffic-hungry mainstream media -- including The Washington Post -- that it vigorously mocks. For his part, Beck, a professed fan of Stewart, has passive-aggressively wished the rally "great success."

But with its Capitol backdrop, exuberant crowd and clever placards, the rally will look a lot like the political theater it sends up. At one point do those mocking hubris actually exhibit hubris?

"It's a tough line to walk," Suzie Colbert, a 46-year-old flight attendant (of no relation to "The Colbert Report" star), said about the balance between sending up the absurdity of American politics and becoming part of it. She said she hoped the rally "wouldn't be too political."

Saturday won't be the first time that comedians have stepped forward as critics of the political system only to find themselves inside the political arena. In Italy, for example, the comedian Beppe Grillo led massive rallies against the ossified and corrupt political culture that proved so popular that they spawned a new political party.

On Friday, Washington teemed with rally-goers wheeling luggage around the streets and toting cameras around their necks, as they piled onto MARC stops at Baltimore International Airport, carried pillows through Union Station, poured out of buses and arrived at the doorsteps of friends who have offered crash pads. At about 2 p.m., scores of rally-goers spilled out of the Smithsonian Metro station, where a banner of the liberal group Media Matters -- which, like Stewart and Colbert unblinkingly scans Fox News for falsehoods, hyperbole and contradiction -- read "Fox Keeps Fear Alive."

On the Mall, workers put the finishing touches on first-aid and family-reunification tents, and electricians tested the half-dozen 9-by-15-foot jumbo screens. The length of the Mall was lined with walls of porta-potties.

In front of the stage, a couple from New Jersey parked a yellow Volkswagen, loaded with dummies of Hillary Rodham Clinton and John McCain in the front seats and President Obama and Sarah Palin in the back. "We're trying to do what Stewart's trying to do: Restore sanity," said Phil McGarrigan, 62, who wore a Bill Clinton mask.

Colbert, the flight attendant, rode her bicycle from Arlington to the stage while clad in an "Obama 08" shirt and wool Obama hat. She had attended the Daily Show taping with the president on Wednesday, but said that she hadn't decided yet what to scribble on signs for Saturday's rally. "Maybe 'Spread the Laughter Instead of the Hate,'" she said. Or something more pointedly anti-Fox. Or something more in the sarcastic Colbert spirit like "I love my Muslim, Foreign Born President," she said.

Colbert, the flight attendant, said she had attended the Beck rally as well, though she found it to be more of a sermon than anything else. But she had a different view of what Saturday would bring.

"Free entertainment," she said.