“I’m not going to get into that,” she said. “It creates a real sour atmosphere at the door.”

And lawmakers are learning the hard way to watch their tongues. At one of his constituent sessions last week, Representative Todd Akin, a Missouri Republican who opposes President Obama’s health plan, remarked that some of his colleagues “almost got lynched” at their town halls. His sympathetic audience laughed and clapped; Mr. Akin replied, “I assume you’re not approving lynchings,” and made a choking gesture. The clip quickly turned up on YouTube, and now the chagrined congressman faces accusations that he was making lynch-mob jokes about Democrats.

Image HEARD In Mehlville, Mo., last week. Credit... Dawn Majors/St. Louis Post-Dispatch, via Associated Press

“I was recognizing the atmosphere as opposed to condoning it,” he said in an interview on Friday. Given the “aggressive atmosphere,” he said, he is turning to the telephone conference as a way to take questions from constituents. “We’re adjusting our format to the situation,” he said.

In some respects, last week’s town halls  fueled on the right by antitax groups backed partly by industry, and on the left by unions  are the logical outgrowth of decades of American political activism. Community organizing is nothing new; President Obama made an early career of it. The civil rights movement, the antiwar movement, the anti-abortion movement, the rise of the religious right  all grew out of grassroots campaigns conducted by methodical organizers.

Accusations of phony grassroots campaigns  “Astroturf,” in Washington argot  also are not new. When Richard Viguerie, the conservative strategist, pioneered the use of direct mail to raise money in the 1970s, he quickly came under attack for creating “the impression of a mass uprising when there were organizers behind it,” said Julian Zelizer, a historian at Princeton University.

But last week’s “town brawls,” as the news media dubbed them, do seem to represent a shift. Instead of each side’s holding rallies and protests, the activism seemed directed personally at lawmakers, with the aim of overwhelming them. Mr. Kratovil, the Maryland Democrat, opposes the health care legislation moving through the House. But he was unable to get his point across, he said. “They simply want to yell when you talk.”

Some might call it democracy in action, but there is a risk. If the pattern continues, lawmakers could grow suspicious, refusing to believe that their encounters with voters are genuine.

“When a politician can’t tell what’s grassroots and what’s Astro, that’s dangerous,” Mr. Zelizer said. “In the long term, that could undermine the potential of grassroots mobilizers to change things. At a certain point, it’s crying wolf. No one is going to believe it’s real.”