I disagree. The Nixon government was awful, but so were many governments before. It wasn't a decline per se, it was just part of the constant change (both good and bad) in leadership.
The real decline began when corporatism started to take over the political/legal system, which was mainly a Reagan phenomenon. Large businesses started pulling everything towards their shareholders, nothing for the workers. Syndicates lost power. The political system started being bought by the corporations. Whole laws started being bought by the corporations.
While corruption has always existed, previously it was something to hide, to be ashamed of. But with the corporate takeover of the USA it gradually began to be normalized, and now it's almost expected from any relevant figure.