Can they while he's still in office, though. I know that he could be impeached if there was the support in Congress to do it but can a sitting president actually be charged and put on trial?
|Judgment in cases of impeachment shall not extend further than to removal from office, and disqualification to hold and enjoy any office of honor, trust or profit under the United States: but the party convicted shall nevertheless be liable and subject to indictment, trial, judgment and punishment, according to law.|
So, yea, he could. It's unclear if he would have to be impeached and convicted first, and then tried in a regular court, or if he could be tried in a regular court without being impeached.