Representative John Lewis, a man who was nearly killed fighting for civil rights, has claimed that Trump is not a legitimate president. While some dismiss this as mere sour grapes, it is certainly an interesting claim and one worth given some consideration.
The easy and obvious legal answer is that Trump’s presidency is legitimate: despite taking a trouncing in the popular vote, Trump won the electoral college. As such, he is the legitimate president by the rules of the system. It does not matter that Trump him denounced the electoral college as “a disaster for democracy”, what matters is the rules of the game. Since the voters have given tacit acceptance of the system by participating and not changing it, the system is legitimate and thus Trump is the legitimate president from this legal standpoint. From a purely practical standpoint, this can be regarded as the only standpoint that matters. However, there are other notions of legitimacy that are distinct from the legal acquisition of power.