Answer:
By spying on an opposition, the presidential candidate can try to disrupt their campaign and win in an unethical way.
Step-by-step 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.