What "clear evidence" was there that Nixon was guilty of this crime?

The correct answer is the tapes.

Nixon's second term as president of the United States ended dramatically when it was discovered that the president had covered up espionage actions against his political opponents, a fact that shook public opinion in the country.

Nixon was re-elected president. During that second term, the scandal called Watergate took place, after a building in Washington where the opposition Democratic Party discovered it was spied on by Republicans. In July 1974, several of Nixon's close associates were accused of involvement in the episode, and in August, the president himself had to admit that he had made investigations difficult. He resigned from the presidency and was succeeded by Vice President Gerald Ford, who used his constitutional powers to forgive him. Nixon died on April 22, 1994, in New York.

1. His resignation has been sufficient punishment.

2. He has been subject to an impeachment inquiry with resulting articles of impeachment which the House Judiciary Committee unanimously endorsed as to Article I (the Watergate cover-up).

3. Prosecution might aggravate political divisions in the country.

4. As a political matter, the times call for conciliation rather than recrimination.

5. There would be considerable difficulty in achieving a fair trial because of massive pre-trial publicity.