Environmentalists, on the defensive after two consecutive setbacks on the Senate floor, last night succeeded in blocking a move that some feared could have weakened protections for the spotted owl and its habitat in the old growth forests of the Pacific Northwest.

The vote of 62 to 34 came after Majority Leader George J. Mitchell (D-Maine) warned that an amendment offered by Sen. Bob Packwood (R-Ore.) would "hamstring the Endangered Species Act." Critics charged that it would have allowed the Forest Service to obtain an exemption for the owl from the act's safeguards.

Packwood and other western senators argued that the Forest Service, rather than being subject to a rigid law, should have more flexibility in deciding what areas should be closed to logging. They argued for a procedure that would have allowed the forest management to weigh the economic and social impact along with the effect on the owl.

The action came after more than a day of debate on the 1991 interior appropriations bill, during which environmentalists narrowly failed to win revisions in several of the longstanding subsidies and concessions to miners and timber interests using the vast, federally owned western lands.

By and large, western senators of both parties united in resisting the changes, but there were exceptions. At the same time, environmentalists formed a novel coalition with fiscal conservatives who are critical of the subsidies that federal land managers give to selected cattlemen, timber companies and others.

Yesterday, the Senate voted 52 to 44 to shelve a proposal to slash almost $100 million worth of road-building projects in national forests.

Late Monday, two senior western Republicans on the Appropriations Committee, Sens. James A. McClure (Idaho) and Ted Stevens (Alaska), led the way in defeating an attempt to suspend a 112-year-old mining law that had been targeted by environmental and conservative fiscal groups.

As that fight came down to the wire, McClure and Stevens indicated they were ready to filibuster to save the old law, which allows miners who develop mineral claims to purchase the federal land for as little as $2.50 an acre. Other westerners buttonholed wavering colleagues as they arrived in the chamber to cast their ballots.

The federally financed roads are used by the timber industry to reach inaccessible parts of forests that have been opened to harvesting. An amendment by Sen. Wyche Fowler Jr. (D-Ga.) would have cut $98 million from the Forest Service's road-building account and would have canceled plans to build 874 miles of new roads.

But the Fowler initiative met a barrage of criticism from McClure and Sen. Mark O. Hatfield (R-Ore.), the Appropriations Committee's ranking minority member.

Hatfield, who is fighting for a fifth Senate term, denounced Fowler's amendment as a "job killer" that would turn "mining towns into ghost towns." But in an apparent effort not to alienate environmental voters crucial to his reelection bid, he also defended his record.

Turning to Fowler with an apparent reference to the South's vast, unregulated, privately owned pine forests, he asked: "What are the 13 southern pine states doing {about the environment}? . . . That's a sorry record."

In yesterday's road-building vote, Republicans turned out heavily to support Hatfield, but they were joined by a number of western Democrats. However, Sens. Brock Adams (D-Wash.) and Timothy E. Wirth (D-Colo.) departed from the western position in voting against tabling the Fowler proposal. On Monday, Sen. Pete Wilson (R-Calif.), in the final weeks of his campaign for governor, sided with mining-law reform. But Sen. Daniel Patrick Moynihan (D-N.Y.), seeking to preserve funds for Seneca Indians in the interior bill, sided with McClure, the measure's Republican floor manager.

Of the vote to table his amendment, Fowler said: "I was very disappointed. When the chips are down, these people who want to cut federal spending lapse into parochialism."

In some respects, the vote indicated slippage in the environmental position. Last year, Fowler had succeeded with a road-building amendment in the Senate, although it was deleted in a conference with the House. Fowler said he had strong support from the Heritage Foundation and the National Taxpayers Union, which have lined up against subsidies to western miners, timber companies and cattlemen.

But Fowler also was going up against the power of key Senate appropriators such as committee Chairman Robert C. Byrd (D-W.Va.), who opposed the road-building cut and mining-law changes.

Last week, Byrd's committee deleted from the interior appropriations bill a House proposal to sharply increase fees charged to ranchers who graze their cattle on federal grasslands for as little as $1.35 per animal a month. The Senate action leaves the matter to be resolved by a House-Senate conference.

Earlier, Sen. Daniel K. Inouye (D-Hawaii) made an unusual appearance before a House-Senate conference on the energy and water appropriations bill to plead for a $5 million allocation to drill geothermal wells in the last tropical rain forest in the United States, on Hawaii's volcanic Big Island.

The conferees approved the funding, despite a pending lawsuit to require an environmental impact study.