In 1974, a special prosecutor found President Richard Nixon had committed felony crimes. But instead of getting an indictment charging him with those crimes, the prosecutor turned his evidence over to the House of Representatives. The House impeached Nixon, and he resigned before going on trial in the Senate.
Since that time, it has been the policy of the U.S. Justice Department that they would not indict a sitting president. However, that is not a law (and it is not in the Constitution). It is just a policy.
And there is currently a lot of discussion happening over that policy. It's happening because of the investigation headed by Robert Mueller into whether Donald Trump conspired with Russia to affect the 2016 election (and various other possible criminal behaviors by Trump).
Should a president who has committed serious crimes be indicted? Or is the only remedy for that a political one -- impeachment and removal from office?
The argument against indicting the president is that he has important duties to perform, and defending himself against criminal charges in a court of law would impede him/her from performing those duties. I don't buy that argument. We expect our presidents to be able to multi-task. In fact, that's a requirement for a good president. And if a president can't perform his/her duties while defending against criminal charges, then they should resign and let the vice-president take over.
Refusing to indict a president puts him in a special category above all other citizens. That's a basic violation of the rule of law. The president is just a citizen -- just like all other citizens of the United States. And the president must be subject to the same laws that all other citizens must obey. If any citizen would be indicted and charged with breaking a law, then any president breaking that same law should also be indicted and charged.
Consider this: If there was indisputable evidence that the president had murdered someone, would you expect the Justice Department to indict and charge him/her with that crime? Of course. So we know that Justice Department policy is just one of degree -- not an absolute. Where is the line between what would warrant an indictment and what would not? Should there even be a line? I say NO -- creating any kind of situation in which a president would not be indicted puts him/her above the law.
Should Trump be indicted if found to have committed a crime? Absolutely. A democracy depends on the rule of law. If the president (or anyone else) is placed above the law, then the rule of law is weakened -- and so is our democracy.