Predictably angry, environmentalists said yesterday that President Carter's approval of completion of the Tellico Dam in Tennessee could cost him reelection support.

A coalition of environmental groups that sought a veto of the energy and water resources appropriation bill also warned that Carter's action may cause him more problems than he bargained for.

Speaking at a news conference, they predicted new assaults on environmental and endangered-species laws as a result of the president's refusal to reject the bill directing completion of Tellico.

The measure, signed into law Tuesday by Carter, also cut funding for several key activities proposed by the president in his effort to overhaul national water policy.

But the central issue was a rider ordering the Tennseess Valley Authority to complete the $145 million Tellico project and freeing it from coverage by any federal law.

The project had been stopped because TVA failed to comply with the Endangered Species Act, which protects the tiny snail darter fish in the Little Tennessee River. The dam would endanger the snail darter.

The veto had been expected on the basis of earlier White House opposition to Tellico. But Carter aides said Tuesday that they feared a veto would unleash congressional retaliation against other important legislation.

Rep. Robert W. Edgar (D-Pa.), an ally of the environmentalists, said the president's decision caused "a feeling of abandonment among some of his friends here in Congress . . . .He has received bad staff advice and it is showing through on this."

Marion Edey of the League of Conservation Voters said that as a result of his decision Carter "cannot be assured of environmentalists' support" next year.

"It is no longer easy to answer if we will drop Carter. He had our strong support in 1976, but, at the moment, there is no guarantee," she said.