Michael D. Barnes, a member of the Maryland Public Service Commission, officially kicked off his Democratic campaign for Montgomery County's 8th District congressional seat yesterday and jazz band music at "old-fashioned political rally" complete with pretzels and beer.

Barnes, 34, drew about 500 county residents and a score of local and state political figures to the party.

"It's politicking Montgomery County style," Barnes said as he and his wife Claudia greeted people at the door of the Bethesda-Chevy Chase Rescue Squad Civic Hall. "It's fun!"

The rally was a fairly unified show of support for Barnes from an often divided county Democratic party. Although Democrats outnumber Republicans in Montgomery County 2 to 1, they have not elected a Democratic congressman in a decade. Two years ago the party factions were split after a divisive primary race among 12 candidates.

Although two other Democrats - Al Mueller and George Berns - are also running for the nomination, the party is solidly supporting Barnes, according to local Democratic leaders. A "dear friend" letter released last week by the Barnes campaign was signed by 65 leaders in the party from all segments of the party.

"This time it looks to me like it's going to be unified," on the congressional race, said Jack Sexton, a former party chairman. Other county races, however, are still likely to divide the party, he said.

But People at the rally were brought together by the fun, if nothing else. Local candidates running for jobs from sheriff to county executive used the gathering to shake their neighbors' hands and renew acquaintances. Also, three Maryland gubernatorial candidates, Baltimore City Council President Walter Orlinsky, State Attorney General Francis B. (Bill) Burch and Harry Hughes, former transportation secretary, were present.

While the 16-piece jazz band played songs ranging from "String of Pearls" to "Happy Days Are Here Again," dozens of children roamed the big room to buy one or two of the 1,500 homemade pretzels, or some fresh-popped corn or a soda.

One 10-year-old said at the end of the party that he has consumed five cokes and four bags of popcorn but the best part of the day was sliding across the floor on popcorn someone else had spilled. While the kids played the adults were busy drinking 25-cent beers.

Although Rep. Morris K. Udall (D-Ariz.) and Sen. Daniel Patrick Moynihan (D-N.Y.) had been expected to attend, they had more pressing business in their home states. But both men sent telegrams of support which Cathy Douglas, wife of retired Supreme Court Justice William O. Douglas, read to the crowd. When Douglas introduced Barnes, the band struck up the theme song from the movie "Rocky."

Barnes, who was apparently enjoying the old-fashioned flavor of the rally, quiped, "You know Rocky didn't win that fight . . . But I do not intend to lose. We can win if we simply get it together."