Interior Secretary James G. Watt, testifying on Capitol Hill on his offshore oil drilling proposals, volunteered yesterday to campaign in 1982 for "those who believe in America" and in President Reagan, telling a House subcommittee that both Reagan's critics and his own will soon lose their credibility.

Rep. Tom Lantos (D-Calif.) had a correction. "If my voice rings with authority," Watt began, and Lantos interrupted: "Self-righteousness," he said. Both men laughed, but that was only one salvo in a morning of artillery blasts over Watt's new policy on exploratory oil and gas drilling on the outer continental shelf.

Testifying before the house environment, energy and natural resources subcommittee, Watt defended his decision to proceed with exploration in the Santa Maria basin off San Luis Obispo, Calif., a controversial tract that environmentalists have said holds little oil but is home to many marine mammals. California Gov. Edmund G. Brown Jr. is expected to file suit shortly to stop the lease sale.

Subcommittee Chairman Rep. Toby Moffett (D-Conn.) unleashed a blizzard of letters, memos and internal Interior Department documents showing opposition to the California action, but Watt said he had not seen some of them and had rejected others. Gov. Brown, he said, "does not have a national perspective and charge the way the Secretary of the Interior does."

Watt said current low figures of 969 million barrels of oil under the Santa Maria basin were only "best guesstimates" and that he had considered Brown's opposition but decided to proceed. The reason, he said, was that he had "determined that the national interests were greater than the state interests" and that offshore drilling had a good environmental track record.