They are the targets of heavy pressure from the Democrats on the upcoming House vote onm the tax bill. "Without them," says a member of the House Republican leadership, "we could not have won" the spueaker victories for the Reagan budget.

They are "absolutely the most vulnerable" incumbents, says a top Democratic campaign strategist. They are walking anarrow line between the demands of party loyalty and the pressures of their home districts, knowing in many cases that redistricting and the instinct for political revenge will make hte 1982 election even tougher for them.

They are not the much-publicized "boll weevils," the conservative southern Democrats who have defied their party leadership to vote for President Reagan's economies. They are the self-styled "gypsy moths," an informal caucus of about two dozen northeastern and midwestern Republicans, mainly from urban areas, who are quietly parlaying their role as a minority of the minority into a strong bargaining position.

The name was coined by one of the members, freshman Rep. Lawrence J. DeNardis (R-Conn.). The gypsy moth is as much of a pest to vegetation in New England and the Great Lakes states as the boll weevil is in Dixie.

But the term also has a political connotation. "The Gypsy moth goes through a unique metamorphosis from worm to fly," DeNardis said, "and I told Michel [House Minority Leader Robert H. Michel of Illinois] that we preferred not to remain as worms but to fly with the leadership -- if we could get some help from the administration on the transition."

The leader of the group is Rep. Carl D. Pursell (R-Mich.), a third-termer from the Detroit suburbs. About 33 members have attended some meetings, with two-thirds of them actively involved. There is no ideological test, but most come out of what once would have been called "the yrockefeller wing" of the Republican Party, moderate-progressive-liberal types who really are regarded as "worms" by some of the more doctrinaire of the dominant conservatives.

Some represent older industrial cities with strong labor and Democrtic organizations -- New York, Philadelphia, Toledo, Providence, R.I., yyoungstown, Ohio, Trenton, N.J., Allentown, Bethlehem, and Scranton, Pa., and Davenport and Dubuque, Iowa.Others represent large concentrations of students and academics in Ann Arbor and East Lansing, Mich., Wellesley, Mass., and New Haven, Conn. A few ar from safe suburban districts in New York, Connecticut of Pennsylvania with progressive inclinations of their own.

Eight of the active members are freshmen and a majority are serving with their first Republican president. While only seven of them wopn with less than 55 percent of the votes in 1980 -- the standard definition of a marginal seat -- many are in jeopardy from redistricting that will force them into districts with more Democratic voters.

Ann Lewis, the political director of the Democratic National Committee, said Friday. "They are absolutely the most vulnerable Republicans, because they represent mature industrial communities and they have been voting against their constituents' interests."

The Democratic ycongressional Campaign Committee has been targeting mailings into many of their districts as a way of softening them up for 1982. Last week, for instance newspapers in the Youngstown, Ohio, area received a release from the "Congressional News Service," the letterhead the Democrats use, saying, "Rep. Lyle Williams (R-Ohio) is on a collision course with his Ohio constituency. The issue: tax cuts -- and who gets them. If Williams supports the plan backed by the Reagan administration, he violates the interests of his own district. If he votes for the Democratic alternative, he violates party discipline."

Despite the pressures and risks, the gypsy moths have been hewing to the party line with remarkable regularity. All of them voted for Gramm-Latta I, the early version of the Reagan budget, and all stayed in line on the key procedureal vote in late June that thwarted the Democrats' strategy of chopping the budget reconciliation bill into six pieces.

When it came time to lock the administration budget cuts into place, two of the moths flew away -- Reps. Charles F. Dougherty of Philadelphia and Claudine Schneider of Providence. But the remainder all voted with the President and the House GOP leadership, making it possible for the GOP to eke out single-digit victories with a couple of handsful of Democratic defectors.

That loyalty was certainly not evident at the start of this congressional session. Early on, the Republicans who were active in the seven-year-old bipartisan Northeast-Midwest ycongressional Coalition clashed angrily with Office of Management and Budget Director David A. Stockman, who rejected outright their claims that the budget cuts would be felt more heavily in their area than in the Sun Belt.

Pursell, as co-chairman of the bipartisan group, said in February that "the proposed cuts, in effect, leave declining regions to their own fate, a policy that in the end will spell economic disaster for the entire country." Rep. James M. Jeffords (R-Vt.), another leader of the group, was so outspoken in his criticism that the White House cut him out of the customary privilege of arranging special tours of the executive mansion for his constituents.

Apparently facing significant defections in Republican ranks, the White House opened negotiations with the conservtive southern Democrats. It was that action, as much as anything, that triggered the gypsy moths to act.

"We got together just before Gramm-Latta I, when the southern strategy appeared," Pursell said. "We expressed our concern that all the bargaining seemed to be taking place with the southern Democrats and we said the Republican cannot afford to write off the Northeast and Midwest just because we hve urban constituents."

Pursell, a former state legislator with a marked preference for the inside game, insisted that the unhappy GOP members discuss their problems with Michel before any confrontations with the White House Michel was immediately sympathetic and supportive.

"He felt their concerns were legitimate," a House GOP leadership aide said, "and he felt he had to solidify the Republican side as much as or more than he dealth with the boll weevils."

Michel arranged for nine of the potential dissidents to have individual appointments with the president, missing an early budget floor vote himself to see that the sessions were going smoothly.

Some concessions were granted immediately; Reagan ordered Stockman to accept Pursell's proposal for one-year extension on an experimental porgram targeting a slice of defense contracts to distressed areas. The president assured all his visitors that if they voted for Gramm-Latt I, there would be an opportunity to negotiate relief on other regionally important programs before the final budget numbers were set. That was enough to produce Republican unanimity on the first test.

"When they saw the president was ready to listen and to deal," said a staff member who works with the group, "it was a great incentive to them to organize themselves." Michel helped again by inviting their comments on the specific budget cuts recommended by the ranking Republicans on the House committees and forwarding their reactions to Stockman and other administration officials.

Eventually, the gypsy moths agreed on a priority package of some $4.8 billion in shifts and restorations, centering on Medicaid, guaranteed student loans, rail and mass transit funds, energy assistance and conservative and the arts and humanities -- all of them programs with special importance for their region.

Stockman, whose role is viewed with suspicion and hostility by many of the moths, "proved to be a bear in negotiations," one of them said, and at times was banished to a bystander's role by Michel. Pursell said, "Our main concern at all times was our role and relationship with the leadership of our party in the House. Dave Stockman is in a different branch of government and many of us resented his trying to dictate to Bob Michel or us."

But Michel was alternately flexible and firm, and in the end, the gypsy moths calculate, they got slightly over half of what they sought -- $2.8 billion of the original $4.8 billion.

All of them stayed with Michel on the key rules test, when the Democrats' effort to deny a straight up-and-down vote on the Republican substitute failed by eight votes. They put renewed pressure on Michel and, in unwitting alliance with the boll weevils, who had objections of their own to different provisions, forced the withdrawal of a further amendment by Rep. James T. Broyhill (R-N.C.) backed by Stockman and the administration. And then with Schneider and Dougherty dissenting, they voted en masse for the Republican package of budget cuts.

A lot of observersf thought they were holding their noses to do it, and some thought they were cutting their own throats. "They're very angry at themselves," said one staff member who works closely with them. "They feel their party has been pulled away from them and that they're voting for things that are disastrous for their people. But they have been under such pressure they're had no choice. The only reason they don't feel like they're writing their own political obituaries is that most of their own constituents aren't aware yet of what they've done.

But the members themseleves express no such qualms and the politics of their position, if dicey, is not without its rewards. Rep. Edward R. Madigan (R-Ill.), the chairman of the House Republican Research Committee and Michel's designated liaison to the gypsy moths reflects the official appreciation for their willingness to stick out their necks.

""Without them," he said, "we would have failed. A dozen of those people are among the most vulnerable of our membership. They are respected on those grounds and they have been given every consideration."

That is not just rhetoric. "The administration has been a delight to work with," said DeNardis, a freshman Republican from New Haven, Conn., and the man who negotiated the compromise on the student loans. In his first six months in office, DeNardis has obtained the city's first urban development action grant, funds for the restoration of the railroad station and the Shubert Theater, and headlines saying, "DeNardis had big role in student loan fight."

Given all that DeNardis said he could take a tolerant view of comments by his liberal colleague, Rep. Anthony Toby Moffett (D-Conn.), that he was "saddened" to see a man with such "distinguished service" in the Connecticut legislature "now embracing Gramm-Latta, which would make phenomenal cuts in the gentlemen's own district."

Rep. Bill Green (R-N.Y.), coming from a silk-stocking liberal district in Manhattan, voted for the Reagan budget and put out a press release claiming he and his colleagues had "turned back anti-North proposals." Green also claimed more specific boons: restoration of a threatened rescission that would have blocked the summer tour of the Joffrey Ballet Company, headquarted in his district, and the rescue of an Economic Development Administration grant for the City Center building.

The existence of the gypsy moths proved a boon even for some Republicans who are far more conservative than most of the members. Rep. Guy V. Molinari (R-N.Y.) of Staten Island was all for the Reagan budget cuts, but when he heard that Michel was meeting with the gypsy moths and arranging appointments with Reagan, he decided to avail himself of the opportunity. As a result, the freshman, who had struck out in appeals to Stockman, got the president to send a Health and Human Services Department official to examine the Public Health Service hospital on Staten Island and gained at least a temporary reprieve in the closing of the facility Stockman had ordered.

Reagan's receptivity to the genuine gypsy moths has made allies of skeptics. Jeffords came around to supporting Gramm-Latta I and II, despite his early condemnation and his reiterated contention that several billion dollars could be shifted from military to social programs. A supporter of renegade Republican John B. Anderson when the 1980 presidential derby began, Jeffords now says, "The people wanted strong leadership and that is what Reagan is providing. I have to respect that."

Schneider, the first Republican from her district in 42 years and one of the two to bolt on the final vote, turned down a personal appeal from Reagan just 15 minutes before the roll-call. "I agonized," she said, "but even with the concessions we were able to get, I just felt that the budget cuts were not in the best interests of my district. They're the ones who sent me here and they're the ones who can keep me here."

To her surprise and delight, Schneider got a phone call from Reagan the morning after the vote, while she was home in Providence, R.I. "He told me, 'If any Republicans give you a rough time on that budget vote, have them see me.'"

Madigan said he had "nothing but admiration" for Schneider and for Dougherty, the second-termer from Philadelphia who cast the only other vote against the Reagan plan. Dougherty will be redistricted into a race with a Democratic incumbent next year and his show of independence won him editorial praise from the Philadelphia Inquirer, which called his vote "a profile in courage."

This show of tolerance is in part a calculated strategy of non-estrangement toward people whose votes will be needed soon on the tax bill. But at both ends of Pennsylvania Avenue, Republican leaders appear to understand that there are genuine risks for some of the gypsy moths in too-close identification with the Reagan policies.

"I think," said one White House official, "there are going to be times when several of those members, for their own reasons, will not vote with the administration." Madigan said, "The tax vote should not be too tough for most of them, but there could be votes that are very difficult. If Stockma tries for another $20 billion in cuts next year, it will be very hard on them."

The Democrats, of course, want to make it just as hard as possible by questioning their voting records. House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) called out the names of a number of the gypsy moths last week, saying the income statisics in their working-class districts make it evident that they shoud be supporting the Democratic version of the tax bill, which few, if any of them, will do. While the Democrats are making calls to key figures in their home districts, the gypsy moths have organized negotiating teams to seek further concessions from the Republicans on the House-Senate budget reconciliation and the pending tax bill.

Molinari reported that phrases originated by the Democratic Congressional Campaign Committee were copied in critical letters to the editor signed by young Democrats in his district.

Sam Fishman, the United Auto Workers' political director in Michigan, said the UAW had endorsed Pursell in his first campaign and had "never treated him as an antagonist. But now he's voting more and more the Reagan line. We're just going to let him dig that hole a little deeper and then we're going to blast him."

State Sen. Mark Kaplan, the Democratic chairman in Vermont, put out a lengthy press release last week accusing Jeffords of "deliberately misleading" Vermonters. "He told us from the start that the Reagan plan was a disaster," Kaplan said, "but he voted for it."

Without sounding the alarm in a way that might make the Republicans even more nervous about voting with Reagan, the Republican Congressional Campaign Committee has inaugurated a special program of training and services for vulnerable incumbents. One of the main focuses of a seminar last week was how to handle this early negative attack. "We want them to know it's coming and that they're not alone in being targeted," said executive director Nancy Sinnott.

From Reagan on down, the Republicans seem determined to keep the gypsy moths off the endangered species list. However peculair they seem to Republican regulars, their votes are needed. Gypsy Moths: Who They Are

These are the active "Gypsy Moth":

Lawrence Coughlin (Pa.), Robert W. Davis (Mich.), Lawrence J. DeNardis (Conn.), Jim Dunn (Mich.), Millicent Fenwick (N.J.), Hamilton Fish Jr. (N.Y.), Benjamin Gilman (N.Y.), Bill Green (N.Y.), Margaret M. Heckler (Mass.), Harold C. Hollenbeck (N.J.), Frank Horton (N.Y.).

James M. Jeffords (Vt.), Stewart B. McKinney (Conn.), Marc L. Marks (Pa.), Carl D. Pursell (Mich.), Ralph S. Regula (Ohio), Marge Roukema (N.J.), Claudine Schneider (R.I.), Olympia J. Snowe (Maine), Thomas J. Tauke (Iowa) and Lyle Williams (Ohio).