From a Wall Street Journal review by Robert S. Merry of the book by Irwin F. Gellman titled “Campaign of the Century: Kennedy, Nixon, and the Election of 1960”:

When Richard Nixon ran against Congresswoman Helen Gahagan Douglas for the U.S. Senate in 1950, his House colleague John Kennedy slipped him a $1,000 campaign contribution from the family patriarch, Joseph Kennedy, who liked Nixon’s anticommunist fervor. Nine years later, when the two men emerged as likely presidential nominees for their respective parties, Kennedy asked Nixon to keep the matter secret so he could avoid a political firestorm from fellow Democrats. The Californian agreed; later, when columnist Drew Pearson got wind of the story, Nixon press secretary Herb Klein denied the report. Nixon acquiesced in the public lie to honor his private commitment to the man emerging as his most threatening political rival.