A funny thing happened to the Young Democrats of Maryland on their way into the era of the under-35 suburban professional.

Once a virtually all-white political network that helped propel the likes of Reps. Steny H. Hoyer and Michael D. Barnes to Annapolis or Capitol Hill, the Young Democrats foundered in the years after the Vietnam War. The organization reached its low point last fall when President Reagan won Maryland with the help of a strong conservative youth vote.

Today, though, YD chapters from Montgomery County to Baltimore are reporting a surge in new members, particularly among college students, minorities and women. Like Democrats of old, they are organizing and recruiting by precinct, but they also are are using new techniques such as mass mailings and seminars on polling and campaign recruiting.

Some experts say this growing interest in politics by people in their mid-to-late-20s is something more than just a demographic curiosity or Yuppie selfishness. Many of these young people, settling into their first jobs or buying their first homes, are putting down roots in older neighborhoods around the state and view politics as a way to affect the crime rate, taxes and development in their own back yards.

"Young people aren't stupid," said Brian J. Berry, 27, a Montgomery resident and statewide president of the Maryland Young Republicans, a group whose ranks swelled last year to 800 members.

"Regardless of what side of the street you're on," Berry said, "you're going to care about pocketbook issues. Young people have something in their gut. And besides, Reagan has changed the whole agenda."

Indeed, as a state adjoining the power center of Washington and one in which Democratic politics vie with the Baltimore Orioles for capturing public attention, Maryland may an imperfect laboratory for testing the attitudes of budding politicos.

"Maryland is the classic example of a lot of young people serving in public office, say, in the House of Delegates or local government," said Rick Reidy, 29, a campaign organizer at the Democratic National Committee.

But, Reidy added, "It's also a place where there's a real resurgence of the need to get involved."

In 1983, the Maryland Young Democrats had 900 members at 10 clubs around the state; today, there are 1,100 members at 22 clubs, 12 'What we're all about is trying to get more people excited and interested in the community, reaching out to people on the bottom, figuring out the best way of operating a campaign.' Victor Weissberg Democratic club chairman of them college chapters, according to group president Vincent DeMarco.

DeMarco, a 28-year-old New Jersey native whose bent for government landed him a job as an assistant attorney general in Baltimore,said the group grew largely through increasing membership at what used to be "lily-white" clubs in Baltimore and Prince George's County.

Montgomery's YD chapter also has grown rapidly, tripling in size in recent months to about 150 members, said chapter President John E. Mennell, who is 25.

In Montgomery, where a strong party structure has given Democrats an unshakable hold on politics and patronage, "precinct officials are telling their sons and daughters to be active, not to sit back," said Mennell, a Republican's son who works in Washington for a Democratic congressman from New York.

Victor Weissberg, 25, has been a precinct official himself since 1978, organizing door-to-door in the Woodside Park area of Silver Spring.

Weissberg, who earns less than $18,000 annually as a member of Barnes' staff in the congressman's Wheaton office, recently was elected president of the Eastern Montgomery-Kensington-Wheaton Democratic Club, one of the oldest and largest political clubs in the county.

"Not every young person is completely self-centered," said Weissberg. "Hopefully, what we're all about is trying to get more people excited and interested in the community, reaching out to people on the bottom, figuring out the best way of operating a campaign."

Weissberg, Mennell and other Young Democrats have applied the rhetoric to some practical problems, volunteering at local soup kitchens or participating in neighborhood crime watch programs. They signed up scores of new voters during the election season last year and are considering a campaign of peaceful demonstrations to halt the sale of South African gold krugerrands at Montgomery coin stores.

Mennell may take it one step further: he plans to run next year for one of three House of Delegates seats in District 20, which includes Silver Spring. The campaign could cost as much as $25,000.

"Young Democrats are just about as progressive today as they were in the '60s," said Baltimore native Kathy Shulman, 25, who coordinates youth programs for the liberal Americans for Democratic Action in Washington.

Shulman, a member of the Montgomery YD chapter, said people like herself "are craving innovative liberal programs.

"We're liberal on social and international issues and probably conservative on economic concerns," she added. "We're trying to build a base to win elections. And we're in it for the long haul."