Power lines may be drawn during the daylight hours in Washington, but when night falls and fine wines flow on the black-tie circuit, bygones seem to be bygones.

"Cappy!" bellowed Secretary of State Alexander Haig at last night's 20th-anniversary dinner of the Americana Project. He was calling to his nemesis of the week, Secretary of Defense Caspar Weinberger. The two men hugged.

If the hottest story in town this week was Haig's feuding with all the "guerrillas" on the White House staff, the second-hottest story was Haig's feuding with Weinberger.

"I was worried about how the differences were being played overseas," said Weinberger, who seemed to be in a particularly good mood. "But it wasn't infighting. Al Haig and I had differences of a policy nature. That was all. We get along fine." Asked if the disharmony hurt Haig's standing in the administration, he said, "His position is strong. The president has great admiration for him."

Haig seemed in an especially good mood, too. But he wasn't quite as loquacious.

"Everything is resolved now," he said. "I have no other comment."

He did open up a little bit, however, in his after-dinner remarks. "My wife, Pat, and I are very pleased to be here tonight with all of you," he said. "As you all know it's been a very dull week for me . . . Whenever my ego soars I just turn on the television . . ." Everybody laughed.

The Americana Project is a lucrative fund-raising program created in 1961 by State Department and White House curator Clement E. Conger to restore and decorate the State Department diplomatic and reception rooms with period furniture and antiques.

Last night's $1,000-a-plate dinner cleared $250,000.

Among the other guests were Chief of Protocol Leonore Annenberg, Secretary of Commerce Malcolm Baldrige, Saudi Ambassador and Mrs. Faisal Alhegelan and Chief Justice and Mrs. Warren Burger.