Democratic and Republican voters in northern Prince George's County and Takoma Park will go to the polls today to select their party nominees for the now vacant 5th District seat in Congress formerly held by Gladys Spellman.

The winners in today's primary will face off in a two-way general election May 19 that could turn into a nationally financed test of Reagan administration policies and the Democratic party's responses to them.

Despite the potential significance of the general election, voter turn out today is expected to be small, a product of the unusual timing of the election and the large number of candidates -- 19 Democrats and 12 Republicans -- on that primary ballot. County election board officials say that at most 25,000 Democrats and 8,000 Republicans -- half the normal primary turnout -- will go to the polls today.

Today's primary will follow regular election procedures, with voting places -- 106 in Prince George's and six in the Montgomery County section of Takoma Park -- open from 7 a.m. until 8 p.m. Results should be known by 10 p.m.

Today's vote caps a month of often frantic activity by the condidates that began last February when Maryland Gov. Harry Hughes called the special primary and general elections to replace Spellman. Her seat had been declared vacant a few days earlier by Congress after medical reports indicated that the popular Democratic congresswoman would not be able to recover from brain damage she sufferd during an incident of heart failure last October.

After weeks of campaigning and expensive media blitzes, the most crowded congressional race in Maryland history had boiled down to a contest involving six or seven well-known Prince George's political figures and a couple of well-financed newcomers.

The Republican primary has been the hardest fought contest, with charges and counter-charges about qualifications and improper behavior overshadowing any discussion of issues. Lawrence Hogan Jr., the 24-year-old son of County Executive and former congressman Lawrence Hogan Sr., was initially considered the easy frontrunner in the race because of his father's name recognition and financial contributors lists.

Hogan's early lead, however, vanished as his toughtest opponent, Bowie Mayor Audrey Scott, focused on his age and lack of experience. In addition, Scott has put Hogan's campaign on the defensive with charges that the older Hogan has used some heavy-handed tactics and his powers as couny executive to ensure that his son wins the primary. Hogan's campaign aides concede that these charges have hurt them and have, in some quarters, turned the race into a referendum of sorts on the actions and record of his father.

On the Democratic side, Spellman's husband Reuben was initially considered the favorite despite his inexperience because of a large anticipated sympathy vote for his wife. But his early lead has been cut in recent weeks by fundraising troubles and the aggressive campaign efforts of several candidates, most notably former Maryland Senate President Steny Hoyer.

Among the Democrats, Hoyer has managed to set up the largest campaign organization and garner the greatest number of endorsements. In addition, he has raised large sums of money and pulled behind him a majority of the county's elected officials who in the past supported Gladys Spellman. Almost all of those officials were part of the powerful Democratic organization Hoyer ran for years until his unsuccessful 1978 bid for lieutenant governor.

Four other Democrats -- Montgomery Del. Stewart Bainum Jr., Bowie Sen. Edward Conroy, Council member Sue Mills and State Sen. Thomas O'Reilly -- have also emerged for the pack of 19 in that party's primary. Of these, Bainum, a wealthy young delegate who recently moved into the Montgomery portion of the district, has become the wild card of the race.

He has spent $50,000 or more of his own money on television and newspaper advertisements that he hopes has pushed his candidacy out of the "unknown" category, but, in the process, Bainum has heard complaints that he is trying to "buy" the congressional seat.

One of the Democratic candidates, Richard E. Lee, 55, of College Park, died of cardiac arrest early this morning at Leland Memorial Hospital, according to hospital officials.