Summer interns mingled with senators last night at the Georgetown home of Pamela C. Harriman, chairman of Democrats for the '80s, for her fifth annual garden party . The party was a benefit for the Senate Majority '86 campaign, and a pessimist couldn't be found.

"After five years of Ronald Reagan, we have more chance than ever," said Harriman. Said Sen. Alan Cranston (D-Calif.): "It's too early to make a prediction, but the party almost invariably does well in an off year. And it's particularly good when the president is a lame duck, as Reagan is." David Johnson, executive director of the Senatorial Campaign Committee, said simply, "We're going to win back the Senate in '86."

Not everyone was talking politics and strategy, however. Most of the 525 guests were between the ages of 20 and 40, and included students, lawyers, Hill researchers and aspiring politicians. "Even Republicans attend," said Maxine Mennen, an employe of the Brookings Institution who was attending the party for the third year. "They will make the donation because it's fun to see and be seen, and it's fun to see the house."

Indeed, the house is a great drawing card. Guests were greeted by uniformed doormen, introduced to Harriman and Sen. Jeff Bingaman (D-N.M.) in the parlor, and then strolled past the living room, laced with objets d'art, into the garden and down to the poolside. Music was provided by local jazz musicians Julia and Co.

An American flag, in honor of Flag Day, was the backdrop for the first buffet table, stocked with fresh fruit, turkey and cheeses. Two ice cream carts were set up in the yard and Harriman interrupted one conversation to ask a volunteer, "Is it okay down there? Are the ice cream cones going okay?"

The party raised $20,000, according to Janet Howard, director of Democrats for the '80s. Despite the $34 requisite donation, however, Harriman said the get-together's primary purpose is not to raise money but to get young Democrats involved. "It is our policy to reach out to everyone," she said. "It's fun to give a party for the young."

And, she asserted, the Democratic Party is "alive and well."