The hour was late at the Rockville union hall and 40 local labor leaders were strangely silent, tired from grilling congressional candidates from Montgomery County on politically sensitive issues from minimum taxes on corporations to U.S. aid to Angolan rebels.

Suddenly, a middle-aged member of the AFL-CIO piped up with the thorniest questions of the night's session.

"You're all quite similar," he said, grinning across a polished conference table at Democrats from Maryland's 8th District, which encompasses the populous eastern half of Montgomery County. "What's the deciding factor? What separates you from the rest?"

The answers, which go well beyond the canned, one-minute replies the candidates gave in hopes of capturing a major labor endorsement tonight, are at the heart of what promises to be the most expensive and intensely fought election ever held in the 8th District.

Seven Democrats, four generally well known by party regulars but with no clear advantage over their rivals, are runners in a summer-long marathon to succeed the locally popular Rep. Michael D. Barnes, a liberal Democrat who is leaving the House to run for the U.S. Senate.

The seven are very much part of the fabric of the 8th, generally homogeneous in their political views but quite different in personal style, temperament and background. Leaders in the Democratic race include state Sen. Stewart Bainum Jr., an ambitious man who is heir to one of the largest financial fortunes in the Washington area; Leon G. Billings, a blunt-spoken candidate who spent 20 years as a staff member on Capitol Hill; Esther P. Gelman, a combative 16-year veteran of the County Council who has championed a number of local issues, and Carlton R. Sickles, a former member of the House of Representatives and popular for his years of work with the Metro transit system.

Also running, but less well known, are John Edward Boehm, a follower of Lyndon H. LaRouche Jr; Wendell M. Holloway, a lobbyist for the Ford Motor Co. who is the only black in the race, and Robert J. Roosevelt, who has unsuccessfully run twice for the U.S. Senate and for the 8th District seat.

The early favorite in the Republican primary race is state Del. Connie Morella, a moderate who enjoys immense popularity in the vote-rich Bethesda area that she represents in Annapolis.

William S. Shepard, a conservative antiabortion candidate from Potomac, is challenging Morella for the chance to return the heavily Democratic district to the GOP, which controlled the congressional seat for more than 20 years until Barnes' victory in 1978.

"Politically speaking, there's a revolution going on," said Michael Grace, a relative newcomer to Montgomery politics.

Grace, a public relations executive and Democrat who lives in Ashton, said he could not yet decide which candidate to support. "It's impossible. Each one has strengths and weaknesses. Each would serve Montgomery County very well."

Even party leaders are loath to declare winners 89 days before the primary vote. "It's anybody's to win," said Lanny J. Davis, a member of the Democratic National Committee who twice ran for Congress in the 8th District in the mid-1970s.

"These candidates have some of the best resumes in the country and there's a huge number of undecided voters," Davis said. "In the end, it all comes down to money."

Money alone does not guarantee success at the polls, but several of the candidates are going to extraordinary lengths to raise it -- and their recognition among voters. In this summer of Maryland's political orgy, with candidates across the state plunging into a limited pool of dollars, Bainum, the fund-raising leader, will travel to California to tap a network of now-wealthy friends from college.

Billings plans to travel to New York to raise funds from national environmental leaders. Morella recently held a successful fund raiser on Capitol Hill that drew several nationally known members of her party.

While thousands of dollars are being raised outside the district, most of the candidates' energies are focused on the 8th itself, a diverse, teapot-shaped area that includes the estates of the rich in Potomac and the tidy bungalows of blue-collar workers in Rockville's Twinbrook neighborhood.

In between is one of the country's most affluent voting blocs, a sedately liberal population that appears quite comfortable with its suburban life style and the public images of its members and past members of Congress such as Barnes and Republican Gilbert Gude.

Eighth District voters want their representative "to be substantive, not afraid to take a position, but balanced, too," said Keith Haller, a Democratic Party pollster and Barnes confidant who is working for Bainum's campaign.

The district may be centrist in its overall attitudes, but not in party primary elections, the threshold votes that will determine Barnes' eventual successor. If the past is any guide, the Democratic primary on Sept. 9 will be dominated by voters from the most liberal wing of the party: federal employes, members of the district's large Jewish population and senior citizens.

Conversely, the absence of many of those voters, who tend to stay away from the polls after the crucial primary, could help Morella if she fends off the energetic challenge by Shepard, a recently retired Foreign Service officer.

Morella would dearly love for the Democrats -- who will have to choose in the primary between the darlings of various wings of their party -- to be in hopeless disarray going into the November election. She could get her wish, given the Democrats' historic penchant in Montgomery County for internal squabbles and backbiting.

Both party's races are just beginning to heat up and the candidates, Republicans and Democrats alike, are largely unknown by the vast majority of voters. That should change in the weeks ahead, during a pre- and post-vacation flurry of literature mailings and advertising.

"Sadly, the race is largely invisible now," said Davis. "That could change soon enough."