He resigned because he'd already had enough trouble as president. Mainly the Watergate scandal. His plan was for him to step down and if he was arrested the vice president which would then be the vice president would pardon him. Also, if he didn't resign he'd likely be impeached anyway.
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Today, people have divided into groups due to diversity.
The amendment process was included in order to ensure that the Constitution did not become weak. This prevented any corruption or loopholes in the amendment process, keeping it from becoming something like the Articles of Confederation. Hope this helps and let me know if you have more questions!