The Senate joined the House yesterday in rejecting an initial effort by conservatives to chip away at the controversial Davis-Bacon Act's regulation of pay rates for federally assisted construction projects.

By 57 to 35, the Senate turned down an amendment by Sen. Jake Garn (R-Utah) that would have let federally funded neighborhood rehabilitation projects ignore existing standards for journeyman-apprentice ratios.

A broader proposal to exempt neighborhood rehabilitation and Indian reservation housing projects from Davis-Bacon's key requirement for payment of locally "prevailing" wages on government construction jobs was rejected last month by the House, 244 to 155.

The Senate and House votes amount to a victory for organized labor, which has lobbied hard to avoid any erosion of wage-protection laws. But conservatives in both houses have indicated they will continue trying to whittle away at the Davis-Bacon Act, charging that the 1931 law is inflationary and outdated.

The Senate is expected to vote later this month on an amendment to lift all Davis-Bacon wage requirements on $1.4 billion worth of military construction projects.

After a preliminary skirmish on this proposal Thursday, the Senate sent the bill to the Labor and Human Resources Committee for hearings on the Davis-Bacon provisions, which will be held next week.

The Senate Armed Services Committee had approved the Davis-Bacon exemption for the military construction bill earlier this year, but the Labor and Human Resources Committee is expected to recommend that the exemption be dropped.

Labor's success in fending off challenges to Davis-Bacon contrasts to the failure of unions last year to win passage of a labor law revision package, which was widely interpreted as a sign of labor's waning influence on Congress.

The Davis-Bacon fight involves a long-standing statute rather than new legislation, and union lobbyists, while clearly buoyed by the Senate vote, were cautious yesterday in assessing their chances for the future.

In arguments yesterday over the housing rehabilitation issue, Garn said his proposal would help unskilled workers get jobs on neighborhood improvement projects by eliminating requirements that work crews be "top heavy" with experienced job-holders. Sen. Harrison A. Williams Jr. (D-N.J.) argued that the existing journeyman-apprentice ratios were necessary to assure good performance.