Gregory Paxton, deputy director of the Georgia Trust for Historic Preservation, doesn't go to many Republican banquets. But when he was invited to a recent Georgia GOP fundraiser featuring Interior Secretary James G. Watt, Paxton seized the chance to lobby politely against "zeroing out" fedral aid to state preservation agencies.

"First I commended Watt for backing the new tax incentives for preservation," Paxton recalled this week. Then he cited the private sector response in Georgia, where 130 projects involving investments of $27.5 million have been submitted in four months, "and noted that the federal administrative cost in the state is just $415,000 this year. Then I asked how this fine program could be continued if the state looses that matching grant."

Paxton did not change Watt's view that someone else can pick up the work load. "But I did stir up some local interest and got the issue mentioned in a GOP newsletter," he said.

That episode summed up the small dollars and large stakes in this current budget fight. Acknowledging that federal aid for brick-and-mortar restorations has ended, preservationists from Maine to Hawaii have mobilized to save the core of the national program by securing at least what Congress appropriated last year: about $21 million in federal support for the states, plus more than $4 million for the National Trust for Historic Preservation, threatened with zero funding for the first time in fiscal 1983.

Bolstered by new evidence of their programs' economic value and cost effectiveness, strategists at the annual Preservation Action lobbying conference on Capitol Hill this week were encouraged by support from key appropriations committee members such as Sen. Patrick J. Leahy (D-Vt.) and Rep. Norman D. Dicks (D-Wash.)

Dicks said that preservation initiatives, such as the effort to re-use Tacoma, Wash.'s, Union Depot, offer vital economic boosts for districts like this, "where unemployment is 13 percent and getting worse."

One key argument, several speakers said, is the nationwide enthusiasm for the preservation incentives in the 1981 tax act. Replacing earlier federal tax inducements that have spurred at least $1 billion in private investment, the new law features a special 25 percent investment tax credit for high quality commercial restoration and rehabilitation of structures that are in historic districts or are on or are eligible for the National Register of Historic Places.

The response among developers, tax attorneys and accountants has been "immediate and very strong," said Sally Oldham, acting chief of the National Register office in the National Park Service. "Preservationists and local officials have also gotten very sophisticated about the tax laws in the past several years," Oldham added.

About 4,000 persons--more than expected-- have paid up to $150 apiece to attend technical conferences on the tax law in several major cities. The final conference, cosponsored by the National Trust, the National Park Service and the National Conference of State Historic Preservation Officers, will be held Monday and Tuesday at the Hotel Washington here.

The conferences have also underscored the new law's limitations. Most notably, the special investment credit is available only when the rehabilitation cost at least equals an owner's adjusted basis in the property. This can disqualify historic structures requiring less rehab work and those whose acquisition cost is very high, as in New York and San Francisco.

While hoping to ease such restrictions, preservation groups are concentrating on preserving the federal funds that enable state offices to carry out national commitments by reviewing tax-benefit applications, surveying possible National Register properties, and evaluating about 180,000 cases per year in which federally aided projects such as highways and redevelopment may affect historic resources.

Preservationists believe that this federal-state administrative partnership should be a model, not a target, of Reagan's "new federalism." They also claim that the administration does not really plan to continue the technical work if the $21 million in matching grants disappears.

According to a National Park Service analysis last December, NPS would need $24.7 million more and a ten-fold increase in technical staff, from 49 to 508, to do the same work in-house. If some reviews were contracted back to the states, the park service would still need increases of $23.9 million and 170 full-time staff. The Reagan 1983 budget would give the relevant park service offices $800,000 more and no additional staff.

That budget, plus cuts and regulatory changes affecting the Advisory Council on Historic Prseservation, have fed fears that the National Preservation program may "soon be too ineffective to accomplish even a shadow of congressional intent," Rhode Island preservation officer Fred Williamson charged this week.

Many in the field are preparing for the worst. The National Trust, hurt by a recession-caused shortfall in private donations and program revenues, announced more budget and staff cuts this week. The Virginia Historic Landmarks Commission, which last fall threatened to stop doing federally mandated work when its grant was held up, is now making contingency plans, executive director H. Bryan Mitchell said recently.

Local groups such as the Hartford (Conn.) Architecture Conservancy are restructuring their projects to attract more private syndication, tap major corporations' investment funds and employ nearly every public housing and development aid program that has survived.

But on philosophical grounds, the growing corps of preservation advocates refuses to retreat. As Rodney Little, Maryland state preservation officer, put it last week, "the national identity" and the proper sphere of federal concern include "not just Mt. Vernon and the Washington Monument, but also the fabric and character of our communities."