Then what is? The Voter has the final say in American elections; is there some authority above the Voters.....? Unless somebody can come up with something else, my answer is the best one: it's the Voters who decide. And, unlike most other Presidents, Reagan's approval ratings have continued to go up
after he left office, and indeed after he died.
He was one of the very few Presidents who actually
did manage to reach across "The Aisle". Probably the jewel in Reagan's crown was that he actually got the Jewish vote. (Jews almost always vote strongly to the left)
Every President since has
said they want to get the two parties working together, but all of them, right up to Obama today, lied.