The League of Conservation Voters gave a qualified endorsement to all six declared candidates for the Democratic nomination for president yesterday and urged environmentalists to support "everybody but Reagan."

The league, a political arm of the environmental movement, said each of the six Democrats has a "fair-to-excellent" record and "would be a vast improvement" over President Reagan, who was described as having "the worst environmental record of any president in history."

Marion Edey, league executive director, also urged environmentalists, who played a key role in the 1980 candidacy of independent John B. Anderson, not to back another third-party effort.

"We cannot afford to waste our time and our votes on a third-party candidate who can't win," Edey said.

Although rating all six Democratic presidential hopefuls as acceptable, Sen. Alan Cranston of California, Sen. Gary Hart of Colorado and former vice president Walter F. Mondale received the most praise in a 47-page review of the candidates' records.

Cranston, described as having "the most consistently pro-environmental record," received the highest rating. He was praised for opposing the confirmation of Interior Secretary James G. Watt, being "an outspoken critic of the administration's environmental policies" and being a "major Senate leader" on wilderness and park issues.

The league's only criticism of Cranston was for his support of water projects.

Hart received only slightly less enthusiastic praise. "He has been active on more issues and has done more to influence environmental legislation than any of the others," the report said.

"Yet, perhaps because he has raised such high expectations, many environmental leaders and lobbyists who have worked with Hart end up feeling disappointed," the report added. "Sometimes he will introduce legislation but not follow through, failing to prepare himself sufficiently to be effective in a committee or Senate floor fight."

Although described as "not a leader on most of our issues," Mondale was praised for having a "fine environmental record as a senator."

The report faulted Mondale for bringing several "unsympathetic" aides into the Carter admistration.

Sen. John Glenn of Ohio received the most negative comments. His environmental record was described as "mixed, often confusing" and "mediocre."

Sen. Ernest F. Hollings of South Carolina has a "mediocre" voting record but has been a major force in legislation protecting coastal areas, the report said.

The league said it is unclear about former Florida governor Reubin Askew's positions on "the full range on environmental issues," but added that he had an excellent record as governor.