Why not read up on it instead of talking to a lying moron like that and asking the stupid question. Here, I will make it easier for you:
All Dershowitz said is the Justice Department said so. He gave zero reasons. The Justice Department is going to try to protect the president who is their boss. Strange you are using a liberal like Dershowitz as your source.
Enlighten us--where is that in the Constitution that a president cannot be prosecuted?