What are some American presidents that can compare to the situations presented in Orwell's Animal Farm?

I think that there could be some comparisons between the way Napoleon and President Nixon viewed power.

Napoleon was far worse a leader than President Nixon. Napoleon ordered executions, tortured members of the body politic, and consolidated his power without any semblance of checks and balances.  Nixon resigned from office because of impending impeachment from the United States Congress.  However, there are similarities between how both men viewed their power.

In many respects, both men viewed political opponents as enemies. For example, Napoleon viewed Snowball as a threat that needed to be neutralized.  When both pigs put forth their ideas for the future of the farm to the other animals, Napoleon does not wait for public opinion to validate which vision is better.  He uses his dogs to chase Snowball away from the farm.  In a similar way, Nixon had an "enemies list."  Along with his staff, Nixon made note of people who showed opposition to his policies and incumbency.  He did not hesitate to refer to them as "enemies."  In conversations with his staff, Nixon insisted on "comprehensive notes" of people that "tried to do us [Nixon's administration] in."  Nixon and Napoleon demonstrated extreme antipathy to people who disagreed with their leadership.

Both leaders also used the government to consolidate their power. Napoleon used his dogs as a private security force.  They followed his orders to protect him.  They also obeyed when he ordered them to kill those who challenged him.  Orwell shows how Napoleon displays his brutality in the way he dispatches his doges to kill the animals who "confess" their disloyalty to Napoleon.  Nixon used the American government in much the same way, according to then-White House counsel John Dean:

This memorandum [detailing the enemies list] addresses the matter of how we can maximize the fact of our incumbency in dealing with persons known to be active in their opposition to our Administration; stated a bit more bluntly—how we can use the available federal machinery to screw our political enemies.

The use of the "federal machinery" included audits from the Internal Revenue Service and handling issues such as contracts or granting federal grants. Nixon did not hesitate to tell members of his staff to use "the Bureau [Federal Bureau of Investigation" and the "Justice Department" against those who stood opposed to his administration.  Napoleon and Nixon did not view power as shared and collaborative.  They viewed it in adversarial terms where anyone deemed as a threat had to be counteracted.