President Donald Trump is now the American President of all of the American citizens, even those who did not vote for him. That is what legitimacy implies. What is ironic is that this is exactly what the Democrat party is trying to take away from him: legitimacy. They tried the same with President George Bush from the beginning to the end of his two terms. Remember that they cannot attack his policies, as they could not attack Bush's policy because the American people has voted for factually both policies. They voted against the Obamas of this world and against Clintons of the world.
The one thing that was left is legitimacy and that is what the anti-Trumpers have locked on to. Attacking his legitimacy. If they attacked his policies, which they would like to, they would be attacking the American people.
What is truly funny is that I was one of the original supporters of Trump. It was me and Ann Coulter, when he first announced, who said he would become president.