John C Calhoun left the vice presidency to become a senator for South Carolina because he

believed that the position of senator would be a more effective platform for advocating and promoting his pro-slavery and states' rights views. As vice president, Calhoun felt limited in his ability to influence policy and shape the political landscape. By becoming a senator, he could have a more direct impact on legislation and use his influence to protect and advance the interests of the South and slavery.