WASHINGTON — Just hours after a national address promising “no whitewash” of Watergate, President Richard M. Nixon privately urged his new attorney general not to appoint a special prosecutor and suggested that a former aide avoid questions by asserting national security.

A series of secret tapes released on Wednesday, the final ones to be made public, shed new light on Nixon’s efforts to stanch the mushrooming scandal in the spring of 1973. On the same night he pushed out top aides and gave his first speech on the episode, Nixon stayed up late making and taking a series of phone calls that planted the seeds for further cover-up.

While he received supportive calls from the likes of Ronald Reagan, George Bush and Billy Graham, Nixon also made a point of talking with Elliot L. Richardson, his choice to take over as attorney general. In the prime-time speech, Nixon told Americans that he had granted Mr. Richardson “the authority to name a special supervising prosecutor.” But now on the phone, he privately told Mr. Richardson not to do so.

“The one thing they’re going to be hitting you on is about the special prosecutor,” Nixon said. “The point is I’m not sure you should have one.” Instead, Nixon said, Mr. Richardson should “assume responsibility for the investigation” himself.