The legislative battle over cut-rate mortgage revenue bonds, the financing technique used by one out of three first-time home buyers, took an important turn on Capitol Hill last week.

The Reagan administration, which long has sought to kill the tax-exempt bonds, announced that it is willing to reconsider its position.

Treasury Secretary Donald T. Regan told the National Governors Association (one of the key groups pushing for continuation of mortgage bond financing) that the administration will "work with you" toward compromise on this controversial issue. "If it can be documented that there is a real need" for cut-rate bonds for first-time buyers, then the Treasury may not insist that they be discontinued at the end of 1983, Regan said.

Prior to Regan's surprise announcement, a major concern among housing groups here had been that the White House would veto any legislation emerging from Congress that extended the federal tax exemption for mortgage bonds.

Support for continuing the cut-rate-bond plan has grown rapidly on Capitol Hill this session, as state, county and city groups have lobbied hard to preserve the only large-scale housing-assistance program for moderate-income buyers.

Mortgage revenue bonds are issued by several hundred local and state governments in 47 states to lower housing costs for residents and stimulate the real estate market. The bonds are now exempt from federal taxation taxes.

Housing agencies funnel the bond dollars to first-time home buyers via long-term mortgages, often at rates three to four percentage points below the going market rate.

Roughly $8 billion worth of such mortgages were made last year, financing more than 150,000 buyers around the country. In some real estate markets, more than half of first-time purchasers during 1982's mortgage-money crunch made use of bond-assisted loans.

However, the existing authority for state and local agencies to issue tax-exempt mortgage revenue bonds will expire at the end of this year.

Because the bonds involve what the Treasury defines as a "revenue loss"--estimated to be anywhere from $100 million to $300 million last year--the Reagan administration has opposed extension of that authority on budgetary grounds.

State and local agencies have vigorously disputed the revenue-loss argument. The Council of State Housing Agencies, for example, estimates that, for every $1 billion worth of single-family-mortgage bonds issued, at least $40 million in net tax revenues are generated. The council says its research demonstrates that every $1 billion in bonds produces cut-rate financing for 19,000 first-time buyers, many of whom otherwise couldn't afford a home.

Those purchases, in turn, reportedly stimulate construction of 15,000 homes and create a chain reaction, inducing purchases of thousands of other homes by "move-up" owners (who sell their units to first-timers).

All told, according to the council, 40,000 homes are financed and sold for every $1 billion in mortgage-subsidy bonds. The economic activity ultimately generates $126 million in federal and corporate taxes, as well as $54 million in state and local tax revenues, the council estimates.

Without the power to provide tax-exempt financing packages for purchasers of single-family homes, a sizable portion of these economic benefits would be lost, according to proponents. That's why state and local governments, the home building industry and real estate interests have mounted what one congressman calls "the fiercest lobbying campaign I've seen in a long, long time" to save mortgage bonds.

Rep. Sam Gibbons (D-Fla.), a member of the House's tax-writing Ways and Means Committee, says he's "absolutely amazed" at the intensity of the pro-bond lobby. Gibbons opposes continuing the bonds. "I'm against any use of tax exemptions for essentially private purposes," he says. But he agrees that cities and states "probably are rattling" the White House into backing off its former, tough position.

"I think the sheer political numbers have now gotten Mr. Reagan's attention," says Thomas W. White, executive director of the Council of State Agencies and principal architect of the lobbying campaign.

"After all," White says, "the folks up there can read tea leaves when they have to." Thus far, according to the council, a new bill preserving the bonds for first-time buyers has attracted 147 cosponsors in the House and 36 in the Senate. Even Sen. Robert Dole (R-Kan.), head of the revenue-raising Finance Committee and a critic of the bonds, is easing his formerly strong opposition.

The outlook for first-time home buyers hoping for discount financing appears to be improving--at least for now.