The Senate's No. 2 Democrat, California's Alan Cranston, joined environmental groups yesterday in calling for the ouster of Interior Secretary James G. Watt on grounds he is "a puppet of the exploiters and destroyers."

In a speech on the Senate floor, Cranston said, "I want Mr. Watt stopped. Now." He charged that during six months in office, Watt "has exceeded even the worst-case estimate of his harshest critics."

Simultaneously, the Wilderness Society published a four-pound reference manual documenting what it called "Watt's Wrongs" and again demanded the secretary's ouster. "There is solid citizen support for the immediate dismissal of James Watt and we today call on President Reagan to do just that," said society executive director William Turnage. In a reference to the villain of the popular movie Star Wars, Turnage called Watt "a sort of Darth Vader of the environment."

Cranston was one of 12 Democrats who voted against Watt's confirmation, but he said at a press conference that more than that number of Democratic senators are now "very concerned" about Watt and would soon speak out against him. The assistant minority leader added that he "suspected" President Reagan might also start reining in Watt, out of concern for public sentiment before the 1982 elections.

Watt, questioned while visiting Tennessee's Great Smoky National Park, responded, "I am an environmentalist. The issue with Sen. Cranston is not with the environment and he has other movtives with his criticism." Watt did not elaborate.

Watt's office said the Wilderness Society had "consistently opposed the Reagan administration and the attack on the secretary should come as no surprise, other than surprise that it is news." t

Both Cranston and the Wilderness Society attacked Watt for expanding offshore drilling leases, reducing enforcement staff at the Office of Surface Mining and halting parkland purchases, as well as for his general effort to open public lands to mineral and other devedlopment. "His proposals do not come from the mainstream of diverse American political views," Cranston said, "but from an anti-government bias so primitive that for a while I had trouble believing Secretary Watt was serious."

Cranston particularly criticized Watt's policies on oil drilling, parkland expansion and land use in California, noting that Reagan had taken out of Watt's hands a controversial decision pending on whether to offer oil leases in four areas off the scenic northern California coast.

That action, he said, "must be considered evidence that they [in the White House ] question his policies."

In "The Watt Book," the Wilderness Society charged that Watt has failed to address air and water pollution issues in the national parks, tried to bypass Congress in decisions involving potential wilderness areas, opened desert and seashore areas to excessive use by off-road vehicles and given away unnecessary fishing rights in the Everglades National Park.