Anything he did while president. If he’s elected, Congress can impeach him (again) and kick him out of office. I fully support this decision, as any state could prosecute any president for any action. For example, Obama drone strikes intentionally killed American citizens without trial, including a minor. They were citizens of Texas. They could use his own words against him, as Obama spoke out against the imposition of the death penalty for John Lee Malvo, one of the Beltway snipers. It’s good to remember that any law you use can also be used against you.