Can a Sitting President be Indicted for Previous Crimes? Posted on 4/22/2018 at 13:43:01 by Bill Crawford, (14)
All of the currently ongoing talk about Cohen's possibly "flipping" on Trump so as to avoid jail time brings up the above question.
Trying to check it out on google, it looks like the answer might be No (See below link); although there appears to be some controversy on the matter.
It looks like the President would have to be impeached and removed from office prior to his being subject to prosecution .
Also, it looks like a sitting President could pardon himself for any alleged crime.
With respect to civil lawsuits filed against a sitting President in a state court. The below-linked article refers to the Paula Jones case (which was apparently pending before the Supreme Court at the time) when discussing whether a sitting president could be subject to a civil law suit.
Checking it out further, I found that the Supreme Court did rule that Paula Jones could bring a civil lawsuit against Clinton.
A related matter. It looks like Trump could pardon , in advance, Cohen of any crime that he might subsequently be charged with.