Fred J. Eckert and Lawrence Coughlin have this much in common: They are Republican congressmen from the Northeast who are running for reelection against attractive, well-financed Democrats.

Beyond that, the similarities of the two House campaigns are few and far between. But together, they illustrate that despite the advantages of incumbency in an institution where more than 90 percent of the members are usually reelected, winning another term is not always easy, even in "safe" districts.

Eckert, a freshman from the Rochester, N.Y., area, is fighting for his political life against Democratic Assemblywoman Louise Slaughter. In 1984, he took over the district that had been held with serene confidence for 20 years by Barber B. Conable (R-N.Y.), now the president of the World Bank.

The district has the look of a Republican bastion, but a combination of an aggressive challenger, a sometimes abrasive personal style and an apparent case of overconfidence have moved Eckert to the top of the National Republican Congressional Committee's list of endangered GOP incumbents.

One sign of Eckert's troubles is that he has alienated one of his best-known constitutents -- Peggy Say, the sister of Associated Press correspondent Terry Anderson, who was kidnaped by an Islamic organization in Beirut in March 1985. Anderson is a native of the district.

For three nights last week, Say appeared in paid television ads in Rochester urging Slaughter's election.

"Nineteen months ago, I went to Congressman Eckert and asked him to speak up for Terry," Say said. "He said 'no.' I think Terry deserves better than that. I think he deserves a congressman who cares, and so do the rest of us."

Coughlin's district, here in the suburbs northwest of Philadelphia, is even more Republican than Eckert's. He, too, is in a tough race -- a rematch against his 1984 opponent, former state legislator Joseph M. Hoeffel (D).

But ever since Hoeffel held him to 56 percent of the vote -- his smallest margin in nine terms in Congress -- Coughlin has been anything but overconfident. He beefed up his congressional staff, hired his first full-time professional campaign manager and raised a record amount of money.

If Coughlin wins reelection on Tuesday, he will be a prime example of a successful survival operation for an embattled House incumbent.

Following the 1984 elections, national strategists for the two parties in Washington looked at the "Any incumbent congressman has to keep an organization in place these days. Those who fall asleep find themselves unelected by the voters."

-- Bill MacDougall, professional campaign manager

results and decided to move first to protect their most vulnerable incumbents.

Martin Franks, executive director of the Democratic Congressional Campaign Committee, said, "We hired general consultants in the off year [1985] to do the essence of a physical on our incumbents covering such things as fund-raising, their congressional offices, constituent services. The consultants came back with prescriptions."

Joseph R. Gaylord, executive director of the Republican committee, said GOP strategists "saw no reason to believe that 1986 wasn't going to be a typical 'six-year itch' election," involving potentally severe congressional losses for the party that controls the White House. As a result, the committee hired full-time staff aides to work with incumbents in five regions of the country and adopted what Gaylord called a "swarming defense" to provide early detection of problems.

The early work of the two campaign committees is another advantage that incumbents enjoy this year. It is expected to be a banner year for House incumbents, whose reelection rate has not dipped below 90 percent since 1974, when the Watergate scandal devastated Republicans.

A key factor is money. The perception of likely success -- which most incumbents enjoy -- leads to campaign contributions from political action committees (PACs) and others and discourages giving to challengers. When an incumbent gets in trouble, as Eckert appears to be in his race against Slaughter, the early warning signs are often found in campaign contribution reports.

In her June 30 report to the Federal Election Commission, Slaughter showed contributions of $ 201,000. That was $ 50,000 more than Eckert had raised and close to the $ 231,000 he spent in winning the open House seat in 1984.

"We weren't raising money like we needed to, and they were raising it by the buckets," said Edward Edwards, Eckert's campaign director, who acknowledged surprise at Slaughter's early fund-raising power.

The Eckert campaign was slow in reacting. In a district in which Conable spent $ 64,000 in winning 68 percent of the vote in his last campaign, Eckert said, "We couldn't get people to take it seriously." Meanwhile, his campaign waited until August to hire a staff aide with fund-raising experience.

One result was that for three weeks this fall Slaughter's negative television commercials assailing Eckert as "Congressman No" went unanswered while the race tightened into what both sides say is a dead heat.

But Eckert's problems go beyond money. Rochester's two daily newspapers supported him in 1984 but have endorsed Slaughter this year. He and his staff complain about biased newspaper coverage of the campaign, citing greater attention paid to actor Richard Gere's appearance for Slaughter than to an Eckert fund-raiser featuring Treasury Secretary James A. Baker III.

The Peggy Say commercial last week was another jolt. Eckert, who says he has done everything possible to help, has accused Slaughter of "exploiting" Say's grief over her brother. He and his aides say there is evidence that the commercial will backfire on the Slaughter campaign.

Nevertheless, they asked Vice President Bush to make a commercial defending Eckert's efforts in behalf of Anderson. Eckert first said he would not use the Bush commercial, which was recorded Monday, but later revised this to say he had "not ruled out using it."

Slaughter said of her opponent, "I don't think he took us seriously at all."

In the Philadelphia suburbs, Coughlin had every reason to take Hoeffel seriously. The Hoeffel challenge in 1984 forced him to use expensive television commercials for the first time in his congressional career.

Before then, the typical Coughlin campaign strategy was to "put up a few signs and shake a few hands on Saturdays," said Kevin J. McNamara, his press secretary.

Coughlin said he did nothing "dramatic" to prepare for a certain second challenge by Hoeffel, but at the urging of Republican strategists in Washington he moved his low-key apparatus into the age of modern politics. Steps he took included:Hiring McNamara, a former reporter who covered the 1984 campaign, as his first full-time press secretary.

Holding, for the first time in recent memory, a town meeting in the district in the summer of 1985. He has held two more since.

Opening a new, part-time district office to provide constituent services to a slice of northwest Philadelphia, an area Hoeffel carried in 1984.

Hiring his first professional campaign manager, Bill MacDougall, who was trained at the GOP's Candidates' Academy in Arlington.

And he never stopped raising money, amassing a campaign war chest that is approaching $ 700,000.

Hoeffel has noticed the differences from 1984, especially Coughlin's fund-raising efforts, his attempts to become more visible in the district and his negative advertising campaign against Hoeffel's record in the legislature.

"To beat an incumbent you have to be a lot better," Hoeffel said. "If you're equal or just a little better, you lose."

Hoeffel said his latest poll showed him trailing Coughlin by eight percentage points but closing in. MacDougall, however, predicted that Coughlin will win with more than the 56 percent he received in 1984, and that from now on he will run hard to remain in Congress.

"Any incumbent congressman has to keep an organization in place these days," MacDougall said. "Those who fall asleep find themselves unelected by the voters."