All the presidents men what advantages could be gained in a presidential campaign by spying on the opposition

By spying on an opposition, the presidential candidate can try to disrupt their campaign and win in an unethical way.

Explanation:

Spying on an opposition leader can provide information on what kind of voters they are targeting, what messages they are sending out, where they will have fund-raising dinners etc.

With this information, a lot of problems can be created. An example of this is President Nixon who spied on his opposition Edmund Muskie.

His unethical tactics including, disrupting fundraisers, false letters and even a completely made up sexual misconduct charge. All of these, severely disrupted Edmund's campaign and paved the way for a Nixon win.

In modern scholarship, all these tactics are considered anti-democratic.