Some may consider it an open question as to whether the Constitution would allow the president to be arrested and tried and convicted in a state court for the state (not federal) crimes of forgery and electoral fraud.
In Clinton v Jones, the president was sued in a civil case by a private party (Paula Jones) under both federal and state law and the Supreme Court held that the private party was not required to wait until the president was out of office before proceeding with the lawsuit.
Nevertheless, in the case of crimes, I would expect the courts to rule that "high crimes and misdemeanors" encompasses both federal and state crimes and that the president must be impeached and removed by the House and Senate before he may be tried and convicted of a serious crime in a state court.
But what if he's no longer President. What if he loses in november? He cannot be impeached then. So, is he just off scott free?
No, you misunderstand. Obama is NOT Constitutionally legitimate, therefore he could technically be arrested as a fraud or impostor. An illegitimate pretender has no legitimate reason to enjoy a Constitutional protocol he does not qualify for. That being said, I don’t expect the Secret Service or Washington establishment to allow Obama to be arrested, but from a Constitutional standpoint, there’s no reason he cannot be.