No one doubted that Frank Mankiewicz, who last month left the post of president at the financially stricken National Public Radio, would land on his feet. But few would have guessed that the loyal Democrat would land in the middle of the four-star public relations firm of Robert Keith Gray, the staunch Republican.

"What we try to do is skim the cream and get the best people," said Gray yesterday. On July 5, Gray said, Mankiewicz will join Gray & Company as an executive vice president and work on lobbying and public relations projects, alongside some other Democrats, Gary Hemel, Betty Anderson and Ronna Freiberg.

Gray was President Eisenhower's appointment secretary and President Reagan's inaugural committee cochairman; Mankiewicz was press secretary to Robert Kennedy in the 1968 presidential campaign and campaign manager for George McGovern in the 1972 presidential race.

After many years as a political operative, journalist and author (books on Richard Nixon and Fidel Castro), Mankiewicz became the chief officer at NPR in 1977. During his tenure the network won many prestigious journalism awards and tripled its audience. But earlier this year NPR officials announced a $5.8 million deficit and this week the results of an audit of the network's finances are expected.

Gray would not reveal Mankiewicz's salary, but said, "I know he's going to land running."

Mankiewicz was out of town yesterday, but was expected back in time for a special fund-raiser for the Robert F. Kennedy Journalism Awards this evening. He'll tell the inside story on the making of "Citizen Kane," the classic film his father Herman wrote with Orson Welles.