The Labor Department today will issue a thick regulatory package rewriting its rules on the Davis-Bacon Act -- the first part of a two-stage strategy through which the Reagan administration hopes to assuage the construction industry while heading off a divisive fight over the law on Capitol Hill.

The 80 pages of proposed rules to be published in today's Federal Register give construction firms almost all the changes they sought in the regulations implementing the 50-year-old law, which makes contractors on public jobs pay the "prevailing wage" in a locality -- frequently a higher rate than they would otherwise pay.

Construction industry spokesmen expressed general satisfaction with the regulatory package, although they were unhappy with a provision limiting the number of lower-paid "helpers" that contractors can employ on federally funded jobs. On the "helpers" provision, the Labor Department gave ground in the face of strong lobbying from organized labor.

But while industry savors its regulatory victory on Davis-Bacon, construction unions, too, are getting something to smile about -- an administration commitment to keep the law on the books. Administration officials argued yesterday that the regulatory changes make it unnecessary for Congress to act on industry proposals for an outright repeal of the wage-setting law.

"We think the result of this is that repeal is going to lose support in Congress," said T. Timothy Ryan, the Labor Department solicitor, who supervised the Davis-Bacon rulemaking. In California, Deputy White House press secretary Larry Speakes expressed the same opinion and said the administration will oppose repealing the law.

Thus President Reagan may finesse a delicate political problem. With the regulatory changes, he can tell business that he "delivered" on Davis-Bacon. But he can still remind his blue-collar constituency that he had honored a 1980 campaign promise to oppose repeal of the law.

The regulations being issued today on Davis-Bacon and on the Service Contract Act, which sets wage requirements on government service contracts, are basically similar to the package the Labor Department sent to the White House for approval two months ago. The only major change from labor's blueprint is in the "helpers" provision.

Other key changes in the new rules will:

Change the way the "prevailing wage" -- the wage that must be met or exceeded on government contracts -- is determined. Current rules set the rate at the wage paid at least 30 percent of the workers in a locality, frequently the union scale. The new rules will generally set the rate at a lower level, at what a majority of workers in the area earn.

* Change the "locality" rules so that high wage rates from an urban area are less likely to be "imported" into a suburban or rural region.

Relax the requirement that government contractors submit weekly payroll records to help Labor enforce the law. Instead, the firms will send in a brief form certifying that they have complied.