When Democrats in Maryland's 28th legislative district talk politics, they talk about the simple, quiet issues that harried candidates must dream about. Club activities, for example. Or about Democratic Del. Joseph Vallario's lawn picnic for 2,000 party faithful last month.

It is not that rural Democrats aren't opinionated. They are. But in the 28th, where the four slate-endorsed incumbent legislators are unopposed in the Democratic primary, there's not much for party regulars to argue about.

Delegates Vallario, William McCaffrey and John Wolfgang and Sen. Thomas V. (Mike) Miller are as closely aligned - and deeply entrenched, politically - as their enormous, predominantly rural district in southern Prince George's County is wide. The group even has a nickname: "the fightin' 28th."

If the motto is reminiscent of an infantry combat squad, it is not wholly inappropriate. This year, the Democrats in the 28th district are banding together to fight what may be serious challenges from several Republican candidates, despite the Democrats' 3-1 advantage in voter registration.

Five Republican candidates are competing for the three delegate places on the general election ballot, and GOP regulars believe that several of them - particularly Ella Ennis, James Panor, and Rodney Sisk - are capable of upsetting one of the Democratic incumbents and becoming the first Republican delegate from Prince George's County in 28 years.

The 28th Democrats, who meet weekly in Miller's Clinton law office to chart strategy, have arrived at uniform solutions for the district's problems - solutions like lower taxes, more municipal services, and greater control of the development of rural areas.

And generally speaking, the delegation can be counted on to vote loyally with the Democratic leadership in Annapolis, the leadership which includes Wolfgang, the chairman of the House Economic Matters committee.

Democrats and Republicans in both Delegate and Senate races differ only marginally on the issues, however. The problem that confront the rural southern area of the county are clear-cut and opinions differ only on the extent to which legislative action should - or could - cope with them.

Not surprisingly in a district where many voters live on large farms or are elderly people dependent on fixed incomes, all candidates in the 28th support the reduction of property taxes. The Democrats would like to see a 5 percent reduction next year in the amount of a home's assessed value that may be taxed.

More immediately, candidates in the 28th stress the problems of continuing development in the district, which includes the northwestern corner of Charles County. Traditionally, the area of the County south of the Beltway and Route 4 has been dominated by farm and rural concerns.

Now, however, several areas of the district - particularly along Indon Head-Highway, Branch Avenue, and the Mattawoman reservoir near the Charles County line - are swarming with developers anxious to build new high-density, bedroom communities.

"The whole area is about to burst into bloom," says McCaffrey. "And there's no way really of stopping it. The problem is trying to control the kind of development that goes on."

So far, many residents feel the development has brought them nothing but congestion and higher taxes. In the last five years, major roads in the district have become among the most crowded in the county as new developments are built along them. Bus service to many of the outlying communities is non-existent.

During 1977, according to state police figures, an average of 97,000 cars a day passed along the Beltway between Interstate 295 and Indian Head Highway, compared to an average of 93,000 in 1976 and 68,000 in 1970.

Meanwhile, traffic south of the Beltway on Indian Head Highway has jumped from an average of 27,000 autos a day in 1970 to 30,000 in 1976 and 33,000 in 1977, according to state police figures.

Tangled intersections abound along the southern corridor of Branch Avenue. One, which links branch Avenue, Route 5, and Old Branch Avenue, has more than a dozen traffic lights and is popularly known as "malfunction junction."

Almost all candidates in the 28th believe that the area's traffic problems will become unmanageable in the coming years unless "major, long-term improvements" are undertaken by state and county roadbuilders.

But even such measures may not be sufficient if large projects like the Washington Gas Light Company's proposed Mattawoman new town near the Charles County line becomes a reality.

Most of the candidates in the 28th district have spoken against of the Mattawoman project. But most were also unsure if anything could be done in the legislature to halt or monitor it.

What may ultimately determine the strength of the Republican challenge to what party members loudly denounce as the "machine" of the Democratic incumbents is the degree to which Republican factions can unite behind the winners of what has become a tough and somewhat bitter House of Delegates primary race.

Panor, 49, an assistant to the school superintendent who ran for the House of Delegates in 1970 and 1974, and Ennis, 38, a legislative aide in the House of Delegates for the last three sessions, are given the best chances of gaining a delegate's seat.

But Ennis, whose husband, Robert T. Ennis, is a former chairman of the County's Republican Central Committee, has antagonized some party workers by supporting Larry Hogan for the GOP nomination for County Executive, according to observers close to the race.

The antagonism reportedly led County Executive candidate Martin Aragona to encourage Mairs, 39, an employee of Ballard and Kronk real estate in Clinton, to enter the race against Ennis.

The fifth Republican in the primary race for delegate is William Crandell, 56, a retired special counsel for the Federal Aviation Administration.

Incumbent Wolfgang may be the most vulnerable of the Democratic delegates. Wolfgang has lost the endorsement of both the county teachers and county police, which went respectively to Panor and Ennis.

Two Republican candidates, Ray Velasquez, 33, a lawyer, and Theodore Higier, 52, a consultant on nuclear energy for Cleveland Illuminating Electric Co., are competing for the GOP slot opposite Democrat Miller in the Senate race.

But, as one Republican worker ruefully put it, "Miller's hopelessly entrenched. If only his family comes to the polls, he'll get 2,000 votes."