Eighty-nine days from now, Democratic voters in Maryland's 8th Congressional District will go to the polls to sample the richest political smorgasbord in recent Montgomery County history.

Democrats voting in the primary election Sept. 9 will choose among seven congressional candidates as diverse as the sprawling district itself: a multimillionaire son of Republicans, a tough-talking westerner, a follower of political extremist Lyndon LaRouche, a formidable female County Council member, the father of a nationally famous black athlete, a self-proclaimed heir to the name and tradition of Teddy Roosevelt and a man first elected to Congress when the others were political greenhorns.

Republicans will probably have a less complicated choice. Constance A. Morella, a street-smart Massachusetts native with proven popularity in her silk-stocking Bethesda district, is expected to face William Seth Shepard, a Potomac Brahmin who holds a Harvard Law School degree and recently retired from the U.S. Foreign Service.

Liberal Republicans such as Morella, a former card-carrying Democrat, once had a lock on the 8th District seat. But in 1980, to the extreme pleasure of freshman Democratic Rep. Michael D. Barnes, the district's boundaries were redrawn to include the liberal precincts of Takoma Park and carve out the conservative strongholds of eastern Montgomery County.

Life for the Republicans has never been the same since.

Which is not to say that the Democratic primary victor will necessarily glide into the seat Barnes is giving up to run for the U.S. Senate. President Reagan performed extremely well in the 8th District in 1984, and many of the most liberal Democrats who turn out in droves in party primaries tend to stay at home on Election Day.

Many observers believe that the general election will go to the candidate who successfully taps the district's concentrated and sophisticated interest groups -- federal employes, Jewish voters, working women and senior citizens.

Morella and Democratic state Sen. Stewart Bainum Jr. already are cultivating those groups to increase their relatively low recognition districtwide.

There is one other crucial variable in both the primary and general elections, and that is Barnes' considerable stylistic legacy. In eight short years, he -- much more than his Republican predecessors -- gave the office a national profile without losing sight of his constituency, from the retired postal clerk in Silver Spring to the defense contractor in Rockville.

Not surprisingly, the seven Democrats are scrambling over each other to say that he or she alone can provide the kind of constituent service and national stature that voters in the 8th have come to expect of their congressional representative.

Similarly, Morella, who will win only if she sparks a mass defection by Democrats, has been careful not to criticize Barnes' record too harshly.

In short, the primaries are practically anyone's to win -- or lose.