It’s not even so much that they they become corrupt. It’s that they become entrenched and as a result they end up wielding power that far exceeds their office. For example, Nancy Pelosi was ludicrously powerful for a mere House member, and Mitch McConnell almost single handedly dictates how half the Senate votes on many issues. A second term for Trump would be the end of America because he controls a huge cult on top of any formal powers he would get from being the President.