Kevin Igoe, the slight, young, unknown Republican challenger in Maryland's Fifth Congressional District, spent several days last week preparing for what he hoped would be a victory in his major face-to-face encounter with Glayds Spellman, the immenseley popular Democratic incumbent with the persona of a concerned grandmother.

He studied her three-term voting record in Congress, which he called abysmally out of touch -- "too liberal" -- with her Prince George's constituents and he collected advice on how to score points against her in a debate. Then, last Tuesday evening, he drove to the northwest Washington studio of WMAL radio for the long awaited call-in show.

Having schemed to force Spellman into a defensive posture in the long-awaited confrontration, Igoe watched in frustration, drumming his fingers on his studio chair, as his carefully laid plans for victory began to fall apart. Before the first few seconds of air time had elapsed, Spellman took control of the microphone to make an "important announcement to the federal employes in Prince George's and the entire metro area," the largest voting block in her district, about a Congressional squabble she deplored that at the very moment was threatening to halt the flow of federal paychecks.

The rest of the show was much the same. Despite Igoe's attempts to go on the attack, his 62-year-old opponent, whose dimpled visage and soothing voice belie a wily political instinct, kept the program primarily focused on discussions of her subcommittee accomplishments, her legislative triumphs and her constituent services.

"It's frustrating," Igoe said after the show had ended. "We just picked a tough one. But you don't get involved with someone as well known as Gladys Spellman without knowing it would be tough."

If Igoe, a 30-year-old former federal worker, seems exasperated that his campaign against Spellman has made little headway and he has been unable to persuade either local or national Republican officials to give him much help, Spellman's previous opponents would understand.

Six years into a Congressional career, 18 years after she first entered local politics as a county commissioner, Spellman is practically entrenched in her congressional seat, a seemingly permanent obstacle to the hopes of young and ambitious Democrats and Republicans alike.

She is, by most accounts, the most popular politician in Prince George's County today -- a fact Spellman, not one for overwhelming modesty, is quite willing to point out -- with nearly universial name recognition and only one rival, County Executive Lawrence Hogan, as a vote-getter.

Since her first race for Congress, a squeaker that Spellman won with a mere 4,500-vote margin over her Republican challenger in 1974, the former PTA activist-turned-politician continually has increased her margin of victory and her hold on the seat. In her last election in 1978 Spellman trounced her opponent with a 55,000-vote margin.

Her dominance of the seat is a lesson in effective use of incumbency and an unceasing responsiveness to the needs of a district that contains the largest percentage of federal workers in the nation.

Like most members of Congress, Spellman sends newsletters that prominently display her name and photo, gives constituents at minimal cost American flags that have been flown over the Capitol mails out hundreds of free copies of government publications, and personally conducts many constituent tours of the Capitol .

Like other representatives she staffs her office with a score of constituent service workers whose only interest is to see that spots in the military service academies are filled, Social Security checks arrive on time and the government bureaucracy is responsive.

The trappings of office also give Spellman, like her colleagues, a decided avantage in campaign season. By virtue of her status Spellman's comments on national issues are given more weight and picked up by the media. People want to contribute their time and money to her campaigns. She is invited to give more speeches than she has time to deliver (unlike Igoe, who has been stuggling to get such invitations), and people are more eager to appear at her events.

As the representative of a district that is a mere 15-cent phone call or a 10-minute drive away, Spellman has been far more involved in local issues than most of her congressional colleagues, and that involvement has been one of the main sources of her strength .

"You're always among your constituents, there's never a moment you don't know what they're thinking," says Edna McClellan, Spellman's top aide who has run all her campaigns. "The Congressman from Iowa can go home at night (during the sesssion) and do paperwork. Gladys goes to speak to groups," With a few exceptions, Spellman spends every evening in her district attending meetings, hosting open door complaint sessions, going to PTAs, Lions Club and other receptions.

"After the 1974 election (where she won with only 52 percent of the vote), we made a committment to get out to the district, to make as much contact as possible," McClellan said. "Complacency is the first step to defeat. We don't just go to people during campaign time. We go to people all during the year."

In addition, Spellman often is called to "fix" local problems that most Congressmen never have to deal with, such as potholes and local county political squabbles. "It comes with the territory," said James Threatte, an aide to Spellman's Fifth District predicessor, Larry Hogan.

There is more to Spellman's popularity than constituent services, good as they may be, or a voting record that follows a politically moderate line apparently palatable to the large majority of Prince George's County voters. a

Like most denizens of the Hill, Spellman caters to the needs of her district's major voting block, federal workers. These are not the "supergrades" who live in Virginia or Montgomery County, but the Capitol Hill elevator operators, secretaries and mid-level bureaucrats who live in conservative suburban areas of Bowie and Laurel and the inner city districts of District Heights and Glenarden and are concerned about new parking fees at federal facilities, cost-of-living adjustments, federal health benefits and the possibly imminent move of their offices to another state.

Like her predecessors in the job, Spellman is aware of how important that constituency is to reelection, and she spends much time praising them and fending off attacks on government bureaucrats by other elected officials interested in making points with their antigovernment constituencies.

In her newsletter, for instance, she runs a semiregular "Beautiful Bureaucrat" column to commend some "wonderfully responsive" government worker and show that bureuacrats "far from slowing down the wheels of government, are really the people who keep them turning."

In order to better serve the needs of her federal worker constituency and do so in a highly visibly way, Spellman positioned herself during her first term on the House Post Ofice and Civil Service Committee, an assignment shunned by most of her colleagues because it is considered low prestige but which nonetheless is crucial in her district because it handles most civil service matters.

In time, she has managed to move up to the chairmanship of the sub-committee that proposes and considers all legislation dealing with employe pay and benefits. The result has been not only high visibility but also a voting record loaded with positions that please virtually every federal worker union in the area.

"Her voting record is 100 percent with us," said Jody Park, director of legislation for the National Association of Retired Federal Employes, which repesents about 410,000 retired U.S. workers.

Spellman carries the same "100 percent" voting record with the nation's largest federal employes union, the American Federal of Government Employes, and receives similar accolades from the American Federation of State, County and Municipal Employes.

Among the stands that have made her the "queen bee" of the federal work force were her out-front opposition to President Carter's controversial pay reform plan that was bitterly opposed by all the federal worker unions, sponsorship of legislation that gives spouses of federal workers a claim on federal retirement benefits even in the event of a divorce, and vigourous opposition to a move by Congress to pay cost-of-living adjustments for federal workers on an annual, instead of twice a year, basis.

While Spellman is known as the federal workers' champion, she also is wary of getting too closely identified with their causes, particularly at election time. "There are those [in the Fifth Congressional District] who work in the federal government and there are those who hate those who work in the federal government," she says, and she quickly will point out the dozens of bills and other legislative acts of hers, that were designed to benefit deaf people, municipalities, users of the Baltimore-Washington Parkway.

In waging his uphill campaign, Igoe, a former budget analyst from Greenbelt has expressed agreement for many of Spellman's votes on federal workers while stressing differences in the areas of defense and federal spending.

At this point, however, not even the local Republican Party has much energy to devote toward a major campaign effort in his behalf. "Who wants to be a sacrificial lamb?" said Larry Hogan Jr., head of the county Young Republicans who recently has toyed with running for elective office.

And Igoe himself doesn't bubble over with confidence. "I think a lot of people see it as a lost cause," he says. "She's just so well known."