This is two months late, but for the sake of posterity, as well as you OP, who might still be wondering the answer to this question, I will try to answer... The smoking gun tape (June 23, 1972) was actually not a priority for the prosecutors handling the case against Nixon and his cohorts (Haldeman, Ehrlichman, Mitchell) until near the end of their investigation. The first bit of damning hard evidence was a tape of the March 21, 1973 meeting in which John Dean outlined the entire Watergate cover-up to Nixon in the oval office, and talked about needing “$1 million dollars” to pay off the Watergate burglars in hush money. Nixon says “We can get that,” making him a party to the conspiracy to obstruct justice . Then moments later, Haldeman (chief of staff) comes into the oval office, and Nixon, who was a lawyer, and Dean, the president’s counsel, instruct Haldeman, who was not a lawyer, how to avoid perjury by saying things like “you can say I can’t recall” to the grand jury, in the event he is called up. This is suborning perjury, which is also a crime. So in the span of a few minutes, Nixon has committed two felonies, and indeed, this tape is the one that made WG special prosecutor Leon Jaworski decide Nixon was guilty, months before ever learning about the smoking gun tape.

Then of course there is the 18 1/2 minute gap, which the prosecutor learned about a couple of weeks before the release of the previously mentioned tape. On this tape, Nixon and Haldeman hold a meeting three days after the WG break in, and, according to Haldeman’s notes, who, if nothing else, was a copious notes taker, talked about Watergate. However, as the tape showed, an 18 and a half minute gap begins right as the conversation turns to Watergate. This was not direct evidence, but, in a trial, would be very suspicious to a jury if not further circumstantial evidence.

Further tapes went missing or were claimed to not exist, that only came out later, such as a phone call between Nixon and his “ax man” Charles Colson that was incriminating. This conversation was thought to be worse than the smoking gun, but by the time it came out, he had already resigned and been pardoned. (I forget the exact contents of this conversation, but can find it if I looked in my notes).

Adding in the “smoking gun” of Nixon’s order to Haldeman to tell the CIA to interfere with the FBI’s investigation, we see a clear pattern of criminal intent. True, an order to have the CIA step in and say this investigation should be stopped seems rather innocuous on its face (in terms of being the evidence that brings down a president) but you have to keep in mind that the CIA did in fact interfere with FBI’s investigation into the Watergate break in, stopping the investigation for two weeks. Further, the news of the smoking gun proved to be more the straw that broke the camel’s back of Nixon’s support in the Senate, than the evidence that actually proved his guilt to the prosecutors.

All of this culminates to Nixon being deeply involved in the cover up of the investigation, which, if he were not president, he would have been indicted and most certainly sentenced for, considering Haldeman, Dean, Colson, and Ehrlichman, all went to jail for their involvement in the same matter. I hope that answers your question.

Source:

Emery, Fred. Watergate. The Corruption of American Politics and the fall of Richard Nixon. 1994. Touchstone.