National Portrait Gallery Director Marvin Sadik-the embattled acquisitor-has seen kinder times.

Sadik has, in recent days, been jilted, threatened, vilified. He's been compared to a plunderer and a Nazi. Because Sadik tried to buy (he has not yet succeeded) seven costly portraits, all by Gilbert Stuart, he finds, arrayed against him, forces as imposing as the mayor of Boston, the senior senator from Massachusetts, and, sharpening his pain, his sister institution, the National Gallery of Art.

Newspaper editorials from Washington, New York and Boston, all mentioning his name, are spread out on his desk. Time magazine is on the phone, it will not stop ringing. Why is Marvin Sadik smiling? I'm resolute," he says.

Sadik's young museum, which could surely use them, would like to pay the Boston Athenaeum $5 million for two famous Stuart paintings of George and Martha Washington. Sadik thought he had a deal. "Nineteen of the 20 Athenaeum trustees have noted for the sale. They are resolute, too," he says. But so is Boston's Mayor Kevin H. White, who does not mince his words.

White, who may or may not run this fall for renomination (he has not yet declared), has sued to stop the sale. mayor White has compared Sadik to Hermann Goering. The Athenaeum's agreement to sell its pair of portraits is "like the Louvre trying to sell the Mona Lisa to the Arabs," observed Mayor White.

"I don't mind being called an Arab. As it happens I collected Islamic miniatures and calligraphy-I love the stuff," says Sadik, who is Jewish. "But then the compared me to Goering, I did not expect the mayor to swing as wildly as that, I always thought he was a shrewd politican. I'm beginning to wonder," Sadik says.

Though Sadik says the Portrait Gallery has spent 14 months negotiating for the Athenaeum portraits, Mayor White contends he first learned of the deal when Gen. James Gavin telephoned him Thursday to call to his attention a story on the sale that appeared that morning on the front page of The Boston Globe.

"Where has Mayor White been?" asks Sadik.

Sadik says the negotiations were conducted between the Smithsonian Institution, the Athenaeum, and the Boston Museum of Fine Arts where the paintings have been displayed, on loan since 1876.

"The Museum of Fine Arts was offered the Athenaeum portraits for $1.5 million four years ago," says Sadik. "Of course they knew they were on sale. They turned the offer down. Howard Johnson, the museum's president, sat in this office and agreed to an arrangement whereby we'd loan both portraits to Massachusetts for one year out of five for the next 50 years. Johnson told us the executive committee of his board had approved that arrangement. He said that he was confident his entire board would follow suit."

Noting "the mayor's quick thinking," The Boston Globe yesterday editorialized, "The case deserves to be heard on its merits and the mayor should be careful that it isn't tainted by charges that he is only acting to serve purely personal ends." The Boston Museum has 35 trustees. Since 1967, one of them, ex officio, has been Mayor White.

Sen. Edward M. Kennedy has called the $5-million sale "a tragedy." "That the Smithsonian hasthis sort of money burning a hole in its pocket should certainly be of interest to the congessional appropriations committees that oversee its budget," wrote Kennedy.

"Congress legislated the establishment of the National Portrait Gallery," says Sadik, who joined the gallery as director a year after it opened in 1968. Its collection was dowdy, its building untested, its mission-show historical portraits of esthetic value-far from easy. Sadik's colleagues agree that in building his museum, he's done a remarkably fine job.

"Many thought the task was hopeless," says Sadik. "For 10 years we've broken our backs-mounted exhibition after exhibiton, published publication after publication. But in the final analysis, no portrait gallery can be worthy of this country unless it has a permanent collection of the highest stature. The possibility that, through some miracle, we could acquire the Athenaeum portraits seemed the fulfillment of our destiny."

Sadik also tried to buy Stuart's last surviving matched set of the nation's first five presidents. For more than a year, he assiduously courted the family that owned them, the Coolidges of Boston. He met with Thomas Jefferson Coolidge IV, the owner of the paintings, his mother, his brother and his aunt. Sadik knew exactly where he'd hang them. Then he learned he'd lost the pictures to the National Gallery of Art.

"The loss saddened me," says Sadik. "The National Gallery already owned fine Stuarts of all periods, 34 in all. It is not as if they needed five more presidential portraits, all replicas at that.

The matched Stuart set has a market value of about $2 million. Coolidge has given the Washington portrait to the gallery. The Jefferson will hang there as a promised gift. The other three portraits, of Adams, Madison and Monroe, were bought by the museum, reportedly for $1 million.

"Am I disappointed? Yes," says Sadik. "Did they belong in the Portrait Gallery? Yes. I'm disappointed in Jeffy Coolidge-but it's a free country. He decided to place them in the National Gallery. I'm disappointed that Carter Brown let him."

Sadik's gallery has won high marks for the quality of its publications and exhibits. Once the weakest sister among the Smithsonian's museums, it now appears ready to compete with the giants. "You keep working, you keep trying," saysSadik. "What else is there to do?" CAPTION: Picture, Marvin Sadik, by Fred Sweets