With the striped tents, the portable toilets, the band and the balloons, it could have been a circus going on in Sandy Bregman's spacious, sloping back yard in Potomac yesterday.

But former vice president and Democratic presidential nominee Walter Mondale was there talking about the Boyds quarry and a proposed dump site in the Travilah Road neighborhood. And Maryland gubernatorial candidate Stephen H. Sachs was there asking Mondale how best to sell a proposed tax increase.

The presence of Mondale, Sachs and dozens of local and statewide candidates was the sure tip-off that yesterday's event was an old-fashioned political picnic, this one Montgomery County style.

Several hundred people paid $12.50 each to enter the Bregmans' back yard, eat chicken and get up close and personal with national figures such as Mondale, Sen. Daniel Inouye (D-Hawaii) and former Indiana senator Birch Bayh, all friends of Bregman, who is running for a seat in the House of Delegates.

Under the hot, bright sun, candidates seemed content to wave rather than shake sticky hands, and the serenity of a neighborhood of horse farms temporarily took on a red, white and blue cast.

"I thought I was going to attend a routine fund-raiser for the state legislature in Maryland," Mondale said as he surveyed the crowd. "Instead, I find that this is obviously a conference of most of the leaders of the free world.

"Maryland has a history of having good government," added Mondale, who said later that he is doing no other local campaigning in politically turbulent Maryland this year. "And I define good government as any government where its leaders in the main supported Walter Mondale for president."

He said he will not get involved in other local races this year because many of the officials who supported him are running against each other. "These endorsements don't mean that much," he said. "It's the people themselves who decide." In 1984, Maryland voters chose President Reagan.

Bregman, who worked in the late vice president Hubert H. Humphrey's 1968 presidential campaign, is running on a Montgomery County District 15 ticket with Dels. Gene Counihan and Judith Toth and Sen. Laurence Levitan.

"For a political junkie like me, it's a marvelous day," said Bregman as she watched more than 600 guests trample her dry lawn and duck under the tents and the trees for shade.

Carlton Sickles, a former congressman who is running again for a seat from Maryland's 8th District, said he attends four or five such events each day of every weekend. "This is the place to be," he said, sweating slightly.

But for all of the not-so-discreet politicking going on in what is usually a very discreet neighborhood, most of the politicians present seemed to take a special joy in poking fun at themselves, perhaps before anyone could beat them to it.

Sachs, who is running an underdog's campaign against Baltimore Mayor William Donald Schaefer for the Democratic gubernatorial nomination, fired off the biggest self-directed volleys.

"The danger in this campaign, as you know, is peaking too soon," said Sachs, who is trailing 2-to-1 in the polls. To slow down the momentum, Sachs told Mondale jokingly, he took a page from the former vice president's campaign strategy and proposed a tax increase.

Then, Sachs said, he chose Rep. Parren Mitchell as a running mate, selecting, as Mondale did in 1984, someone from "an underrepresented group." Mitchell is black. Mondale's 1984 running mate was then-Rep. Geraldine Ferraro (D-N.Y.).

"But we're still going up in the polls," said Sachs. "So I came here to say, 'Help me Fritz. Please help me.' "