The construction industry has shifted tactics on the Davis-Bacon Act, which sets the minimum wages on federally financed construction projects. Instead of trying to have it repealed outright, employers are chipping away at the 50-year-old legislation, trying to limit the kinds of projects to which it applies.

The idea is to go after the separate bills that come up routinely each year authorizing various federal construction projects. Industry lobbyists hope to attach amendments to each of these, waiving Davis-Bacon for the projects involved. That approach is expected to be tested first in the 1982 military construction bill authorizing $4.l billion in projects in this country. The version that has been sent to the floor by the Armed Services Committee in the Republican Senate already carries a rider waiving application of the 1931 law.

Yesterday, as the House approved its version, Rep. M. Caldwell Butler (R-Va.) briefly offered a similar rider. He later withdrew it.

Industry lobbyists say Davis-Bacon costs the taxpayers $1 billion a year in excess wages. But labor disputes this; it regards the Davis-Bacon protection as necessary and near-sacrosanct.

Largely because of labor's attitude and a campaign promise by the president not to seek repeal if elected, the Reagan administration, now struggling to find ways to reduce military and other federal spending, has been ambivalent.

Labor Solicitor Tim Ryan said his department "is against repeal of Davis-Bacon." But he also observed that, in the present economic and political environment, those fighting to eliminate the act "have a lot of things going their way."

"No one from the administration has told us to back off, or has indicated opposition" to the idea of waivers, said Richard T. Haas, spokesman for the Washington-based Associated Builders and Contractors Inc., an industry lobbying group representing 16,000 mostly non-union construction firms. It has long opposed Davis-Bacon enforcement.

The Davis-Bacon law originally was designed to block the use of cheap, presumably unskilled labor by itinerant contractors bent on winning federal construction jobs by underbidding the usually local competition. The law sets a "prevailing wage" determined by the Labor Department on a job-by-job basis.

Critics charge that the "prevailing wage" is frequently construed to be the highest union rate paid in an area, which they say unnecessarily adds to federal construction costs. Defenders say that the "prevailing wage" as defined for Davis-Bacon purposes often is lower than union rates, and that workers would be vulnerable to wage-gouging by unscrupulous contractors without the act's protection. Both sides cite studies, often the same studies, to support their claims.

The Labor Department last month tried to appease the combatants by offering administrative "reforms" aimed at eliminating the law's objectionable parts, while preserving the law itself. In 80 pages of proposed rules published in the Aug. 14 Federal Register, the department called for redefining the act's "prevailing wage" requirement to reflect more accurately each area's actual construction wage scales; allowing contractors to hire one lower-paid "helper" for every top-scale journeyman, as opposed to the current requirement of seven journeymen to every "helper," and abolishing wage-reporting rules intended to help the government enforce the law.

The suggested compromise angered both sides. Robert A. Georgine, president of the AFL-CIO's building and construction trades department, said the proposed administrative changes would "gut the law." ABC Inc., the industry lobbying group, said that, even with the administration's proposals, "The Davis-Bacon Act will remain a costly burden on the American taxpayer until it is finally repealed."

The administration's peace offering came in the midst of the waiver war that already had begun to yield promising results for the construction companies. At their urging, and with the help of such allies as Sens. John Tower (R-Tex.), chairman of the Armed Services Committee; Strom Thurmond (R-S.C.), chairman of the subcommittee on military construction, and Senate labor subcommittee chairman Don Nickles (R-Okla.), the Armed Services Committee on June 18 amended the 1982 military construction authorization bill to waive Davis-Bacon requirements.

Thurmond claims that the waiver, which would be in force for the duration of all domestic military building projects authorized by the bill, could save the government $400 million. Labor sources say that estimate is groundless.

The measure awaits Senate floor action. Meanwhile, Georgine and his supporters are fighting back with some lobbying of their own. An ad in the October issue of the Columbia Journalism Review, which also ran in Tuesday's Washington Post, asks journalists to take a closer look at organized labor's position on Davis-Bacon and to dismiss the "blind foolishness and blatant propagandism" of industry lobbyists intent on destroying the law.

Still, industry forces opposed to the act are in a noticeably upbeat mood nowadays.

"Labor recognizes that if we ever get that first nail in the coffin, we could tack the lid down," said Rep. Tom Hagedorn (R-Minn.), ranking minority member of the House economic development subcommittee, and a foe of Davis-Bacon. "With the president saying that we need to...get a handle on the budget, support for this act is diminishing rather rapidly. We're a lot closer to repeal than we've ever been before," Hagedorn said.