Ken Starr may believe that a sitting president can be indicted, but he really has no clue and would have been overstepping his authority. ---A sitting president cannot be indicted, he can only be impeached. ----Impeachment of a president is the only Constitutional remedy to remove a president. Even if it were so, an indictment would be stayed until after a president left office and would never pass muster anyway since presidents have the constitutional authority to pardon themselves with or without charges. Folks need to bone up a bit on the Constitution. For example, the other day I heard an MSNBC contributor say that Illegal Aliens were having their Constitutional rights violated-----Wah Huh??
Allowing any sitting president to be indicted seems to contradict the Constitutional scheme. The sole remedy provided for removing the president is impeachment. Not indictment. As Buy1Sell2 pointed out above, all prosecutions must originate in the Executive Branch. That would be the same Branch the president controls. He can stop any prosecution in its tracks, even a special counsel. The congress is not impotent against a lawless president, they have impeachment. Similarly a state cannot be allowed to prosecute a sitting president. The potential for political abuse plus the potential for a nasty state-federal standoff, show what a terrible idea that would be.
Trump needs to watch his arse though. We can argue that Starr was wrong about being able to indict him, but Starr can rightfully point out that he nevertheless caused Clinton enough heartburn to get him to enter a guilty plea, be fined, and lose of his law license for a number of years- all while he was a sitting president. Of course, Starr had him cold lying on tape. Mueller's problem is that he doesnt have anything on Trump, at least related to what he was charged with looking at. If he can get Trump into a perjury trap, he can create a crime. Message to Trump: Don't do that.