Mayor Kevin H. White wants a cease-fire in the gentlemanly war of invective over who gets to hang on to George and Martha-Boston or Washington.

He proposed a six-month moratorium yesterday on negotiations for the planned $5-million sale of the famous Gilbert Stuart portraits of George and Martha Washington to the Smithsonian Institution's National Portrait Gallery. He said that would buy time to raise enough cash to keep the paintings in Boston.

The mayor also called in political shock troops to lead the public fundraising drive to help keep the portraits in Boston.

Mayor White and Sen. Edward M. Kennedy (D-Mass.) have banded together to block the sale by the financially troubled Boston Athenaeum, owner of the porytraits, in a retaliatory move Kennedy dubbed, "S.O.S.-Save Our Stuarts."

That Battle cry, said Kennedy, speaking at a press conference at the Museum of Fine Arts and standing below the portrait of the nation's first president that graces the front of the $1 bill, "is a message that will ring out over the countryside."

The cultural war also has drawn allegiance from former Speaker of the House John W. McCormack, Massachusetts Lt. Gov. Thomas P. O'Neill III, son of the current house speaker, and from the publishers of both of the city's daily newspapers.

Kennedy's support for White's campaign is considered of the tension between them since the mayor said he lost the 1972 Democratic vice president nomination because Kennedy refused to support him.

The fund-raising campaign will raise money for the Museum of Fine Arts to purchase the portraits from the Athenaeum.

"About $2.5 million ought to do it," said White, "plus some sweets."