Within hours of the announcement that the Congressional seat held by ailing Rep. Gladys Spellman was up for grabs for the first time in seven years, the host of politicians and dreamers who populate Prince George's County were plotting their rise to Capitol Hill.

"I've never been one to hesitate," said Bowie Sen. Edward T. Conroy, who announced his candidacy at 9 p.m. Friday, only four hours after House Speaker Thomas P. O'Neill announced that he would be declaring Spellman's seat vacant because of her illness.

"In all likelihood, I will be a candidate," said Lawrence Hogan Jr., the 24-year-old scion of the Republican county executive. The senior Hogan, who held the seat for six years, had sworn off the race but was urging reporters to call his son.

"Is Conroy holding a press conference?" asked former state Senate President Steny Hoyer, who was using the weekend to take the county's political pulse before deciding whether to add his name to the list of aspirants.

In every corner of the sprawling county, the old-time pros, the young hustlers, the has-beens and campaign junkies by the dozen were claiming to be consulting with their families and their egos and taking dozens of phone calls from supporters clamoring for their public service.

"Add my name to the list," said Davis Tomasin, an unsuccessful Democratic primary challenger to Spellman in 1978. "The guy who had the guts in '78 is gonna do it again."

Meanwhile, Reuben Spellman, who for years remained the quiet, shy companion of his political spouse, held a press conference to declare that he, too, would be entering the race for the 5th Congressional District seat.

"I want to carry on her work, and not just go for a free ride," said Spellman, 71, who seven years ago held an umbrella over his wife as he silently watched her make a similar announcement.

Spellman had hoped to emerge as the consensus Democratic candidate in the special primary election this spring, but by yesterday afternoon, all hope of a quiet or bloodless succession seemed to have been shredded by the whirlwind of competing ambitions. "It could very well be a brigade charge," conceded Conroy, who quickly added, "but some of my advisers say that that may only increase my advantages."

Both Republican and Democratic campaigners were predicting, in fact, that the campaign could set new standards for drama and spending in county politics as national organizers moved in on an open seat in a year of slim Congressional margins.

"The general election is inevitably going to bring in enormous national attention and national pressures -- it will become a test for both parties," predicted County Council Chairman Parris N. Glendening, who has been gearing up for the county executive's race next year but said today he might shift gears and enter the special election. "We're talking about $100,000 for the leading Democratic primary candidates and an expenditure of half-a-million on each side in the general."

Because the campaign is likely to be very short -- Gov. Harry Hughes could limit it to 70 days -- county politicians said yesterday that the candidates with firmly established name recognition and cash sources were likely to have a powerful advantage, especially if the field grows large.

Among Democrats, that circle includes Conroy, Hoyer, Spellman and perhaps half a dozen other officials, most of whom have said they will not run. In the Republican party, which has far fewer registered voters and traditionally less fractious primaries, Hogan Jr., former County Councilman John Burcham and Bowie Mayor Audrey Scott have the strongest political bases. Scott and Spellman's little-known 1980 general election challenger, Kevin Igoe, announced they were in the race yesterday, while Burcham and Hogan were still officially thinking it over.

Still, a lack of money or name recognition was doing little to discourage a hatful of pols who let it be known yesterday that they could be persuaded to sit in Congress.

"I'm sitting down with my family this weekend," said State Sen. Arthur Dorman.

"In terms of electability," said State Sen. Tommie Broadwater, "it's number one Steny Hoyer, number two Conroy and number three Broadwater."

"I wouldn't be being honest if I didn't say it had crossed my mind," said former county council member and state senator Roy Hart, who ran unsuccessfully for the seat against Hogan in the early 1970s.

"A number of people have talked to me and I've considered it," said state Sen. Thomas O'Reilly. "My campaign committee met with me in December about running," said state Del. Kay Bienen. "We could put it together to do it, and if it were a big field I could probably win."

But while others hinted of possible candicacies, Reuben Spellman made his claim bluntly. "My concern that this district be fully represented has made necessary this announcement today," Spellman said during his first solo encounter with the media.

Spellman, who admitted he had little of the political experience of some of his expected opponents, said he was seeking the seat in order to continue the programs and philosophies of his wife, who has remained semicomatose since suffering heart arrest during her reelection campaign four months ago.

"My platform is simply what I have already stated -- that next to Gladys herself I can best carry out the overwhelming mandate given her Nov. 4," he said.

Spellman's willingness to run for the seat surprised many family friends and political acquaintances who have known the former engineer and retired government employe as a retiring man who was frequently exasperated by the political world in which his wife blossomed. Said his daughter, Dana Spellman O'Neill: "Where my mother flowers in front of a crowd, my dad gets nervous, but once he gets used to the spotlight, he'll be fine."