SUN LAKES, Ariz. >> It was no accident that Gina Gennaro found herself outnumbered at a town-hall-style meeting the other night and shushed when she tried to press her objections to Republican plans to remake Medicare.

Throughout the two-week congressional recess that ended Monday, at public meetings across the country, a running skirmish was fought among grass-roots activists from both parties. They wanted to duplicate — or prevent — the kind of angry confrontations, many of them organized by Tea Party supporters, that occurred at such meetings during the heat of the 2009 debate over the health care overhaul.

This time, the focus was on budget cutting. Who won the skirmishes depended on which side filled the most chairs. And supporters of both parties were taking part in the maneuvering.

“We need you to show up in force to make sure the far left doesn’t drown out the debate!” Tea Party activists said in an email alert intended to fill as many seats as possible at a gathering that Rep. David Schweikert, a freshman Republican from the Phoenix area, held Wednesday night in a police substation in Tempe, Ariz.

The email said the liberal group MoveOn.org was at work lining up liberals to stack the audience. Indeed, MoveOn.org is one of numerous groups across the political spectrum that have been busy organizing partisans to attend public meetings, but Schweikert still faced a generally supportive audience. Generally.

“It felt like it was me and a couple of others against everyone else,” said Gennaro, an artist and an independent, and one of a handful of critics in the audience who grilled Schweikert. “Next time, I’m going to bring more people with me.”

Every public meeting has similar nuts and bolts: the chairs, the sound system and, increasingly, with the debate over budgeting at a fever pitch, the big screen used to blow up multicolored pie charts and graphs showing government spending gone wild.

The big variable determining whether a meeting will resemble a graduate policy seminar or degenerate into a shouting match is the makeup of the audience.  

Who will fill the chairs, and spring out of them to take the microphone and comment vociferously on the overhead projections, is no accident, as a glimpse behind the scenes at several meetings in Arizona last week made clear. Long before the member of Congress in question strides out in front of a crowd, activists are at work trying to shape the crowd.

Gennaro was attending her first public meeting, and she said she showed up on her own, without any activist group contacting her. She acted like a veteran, though, throwing her hand into the air often to be called upon, offering hard-edged questions and comments and, at the end, ignoring altogether Schweikert’s protestations that the event was over as she continued her critique of Republican proposals to reduce future spending on Medicare.

At another public meeting just down the interstate on Friday morning, Rep. Jeff Flake, another Arizona Republican, found a friendly crowd at the community center of a well-off retirement community known as Sun Lakes.

But there was plenty of behind-the-scenes dialing to fill the seats and shape the audience beforehand. The meeting was officially a campaign event for Flake’s fledgling effort to replace Sen. Jon Kyl, a Republican who is retiring. After hiring a firm to contact voters with electronic robocalls, the campaign informed the Sun Lakes Republican Club of the event just a couple days beforehand, leaving far less time than the political activists there were accustomed to turn out a crowd.

“I just found out about it,” Michael Tennant, the club chairman and a retired construction equipment salesman, said on the eve of the meeting. “I’m doing all I can to fill the room, but it’s on a Friday. That’s the day people are playing golf, getting ready to go out to dinner or resting by the pool. They might be at a class or singing at the church choir, and many people still have their grandchildren around from Easter.”

Still, through a barrage of emails to club members and phone calls to the 30-40 members who do not use computers, he was beaming that the room was packed with Republicans.

One of them was Richard D. Fife, 79, a retired electrical engineer, who considers the country off the rails and who raised his hand high when Flake asked, as many Republican lawmakers have been asking, whether anyone in the audience feared that their children and grandchildren would have less opportunity than they had.

“It’s our turn to get out of our seats and into the streets,” said Inez McGee, a retired teacher seated nearby, speaking of older Americans who have the free time to become politically engaged.

A onetime Democrat who is now an active member of the Republican club and “emotionally connected” to the Tea Party, McGee sent emails to about 40 friends inviting them to the meeting and was pleased to see that many had joined her.

Over at the Democratic club, emails had also gone out. “This is the first I’m hearing about it,” Ruth Kloner, the group’s publicity chairwoman, said one day before the Flake event. But Kloner contacted her fellow Democrats and discovered that efforts were already under way to notify opponents of the congressman to get them in the room as well.

Flake, who faced some skepticism in the audience but little in the way of protest, said the public meetings that centered on President Barack Obama’s health care proposals were far more heated than anything he experienced this time around. Back then, it was Tea Party activists who were emphatically telling Democratic lawmakers to lay off health care. Now, the tables have turned.

“There’s more organizing going on now,” Flake said. “There’s hiring of people to come in and stir things up. If someone is in there to agitate, they’ll do their job. You can’t do anything about it.”

In his PowerPoint presentation, Flake made clear that none of the older Americans in the retirement communities he visited had reason for alarm.

“If you qualify to live here in Sun Lakes, this plan will have no impact whatsoever on your retirement benefits,” his presentation said in Sun Lakes. An hour later, in Pueblo Norte, a community farther north in Scottsdale, a new slide was in place reassuring residents there.

Members of Congress began preparing for the public meetings weeks ago, in anticipation of tough questions they would face, according to legislative aides. There were at least a dozen strategy sessions on the budget with members in which Rep. Paul D. Ryan, the Wisconsin Republican who introduced the party’s budget, and his staff explained the proposal and how to sell it.

Still, there are always unexpected moments. After hearing Flake talk of how older Americans were living longer and how their health costs were growing, one woman remarked that maybe she and the others in the room were living too long.

“I’m glad,” Flake said of the increase in life expectancy, “and I’m sure all of you are as well.”