Republican moderates in Congress - sensing they may hold the balance of power in the crucial upcoming battle over the fiscal '83 budget - are on the move. President Reagan's 1981 string of economic-issue victories would be snapped in early 1982 if the GOP moderates, called Gypsy Moths, broke ranks with the White House and their Capitol Hill leadership.
So far the President has overcome the Democrats' 26-seat lead in the House with the help of 25 or 30 conservative Democrats, dubbed Boll Weevils, plus the unprecedented, near-100 percent lock-step unity among Republicans. Any serious defections among the 20 to 30 Gypsy Moths could end Mr. Reagan's hopes of further cuts in the next budget. That budget, now in the works, will set the stage for the new session's congressional battles in January and next fall's elections.
The Gypsy Moths are near bolting over what they claim are Reagan excesses in defense spending, tax breaks for industry (particularly the energy industry), and spending cuts that hurt people.
''We have come to the point where we may have to take the last stand - and it may be on fiscal year '83,'' says Rep. Harold C. Hollenbeck (R) of New Jersey. Mr. Hollenbeck was one of three Gypsy Moths who broke ranks with their party last week on the 1982 budget ''continuing resolution.''
''We're at a time now economically when these people programs they're cutting are needed,'' Hollenbeck told the Monitor. ''They're not taking any kind of a rational or responsible look at the defense budget. I'm going to continue to vote against cuts in programs I deem necessary, until I see a responsible White House action toward defense and toward closing loopholes created by the Kemp-Roth tax plan, and strengthening existing windfall profits tax on big oil.''
''Party unity means nothing when you don't have a party left,'' Hollenbeck asserts. ''The party was torn asunder in the Northeast and Midwest by this Sunbelt attitude (in the White House).''
The GOP moderates have less to fear in next year's elections than do the conservatives, says Iowa Gypsy Moth Jim Leach, chairman of the Ripon Society and regarded by some as one of his party's thoughtful young spokesmen. Leach argues the precedent for the 1982 elections could be 1974, when only one of the 45 Republicans who lost was a moderate. The losers were conservatives.
''The people who get defeated when the American body politic are upset are those who are identified in the direction of the extremes,'' Leach says.
''Nobody elected President Reagan to preside over a recession,'' Leach told the Monitor. ''As the recession deepens, to a degree his leadership weakens. You're going to see a larger, increasing moderate constituency in Congress that should have greater sway - if not over the exact voting process, then over the decisions voted upon.
''To some degree, the Republican Party is now identified as jeopardizing the security of the elderly, offending the young on environmental issues, as aggressive abroad, alienating blacks and alienating labor,'' says Leach. ''Among constituencies, there aren't very many people left a base could be built upon.''
The majority of Gypsy Moths would prefer to cash in their influence behind the scenes, to persuade the administration to temper programs so they can preserve party unity on the House floor. A dozen Gypsy Moths were scheduled to meet with the President's top White House staff Dec. 15.
''We're trying not to be naysayers,'' says New Jersey's Hollenbeck. ''We're trying to have some input into the process rather than go home for Christmas and come back in late January or mid-January and find we have a budget package already in place.''
One reason more Gypsy Moths did not bolt last week was to preserve a working relationship with the White House. And some felt blocking a three-month continuing resolution was not the right occasion for a split.
''There were a number of very reluctant yeses,'' says Hollenbeck.
Many Democrats share the view that the moderate Republicans will likely hold the balance of power in the next session.
''It's unlikely Reagan will continue to roll along,'' says Rep. Donald J. Pease (D) of Ohio, a member of the House Ways and Means Committee and of his party's whip organization. ''If the cuts he proposes for '83 are anywhere as deep as they're rumored to be, there will be 20 or 30 Republicans defecting. I think it's unlikely more than 30 Democrats will defect. And that means that probably he's not going to get his way.''
Mr. Pease holds out one caveat - that the deficits for fiscal 1983 could be so ''gargantuan'' that Gypsy Moths may feel driven to make more spending cuts.
The Gypsy Moths are closely allied to the mainstream Democrats in many of their concerns. They are opposed to natural gas deregulation, even with a windfall profits tax applied.