There are legit things that if singled out look 'bad', 'swampy', 'other'.
So the question is 'is that enough justification or reason to demand and push for an investigation'.
If Hillary announces she is running and end up against Trump or even Romney is it ok for them to call for an investigation into her emails again? It still looks wrong, despite being cleared. So they can just say they want a new investigation or a different take on it since it still looks bad to them.
The stupidest of voters fall into that trap. Yes 'but, BUT, it does look bad, so lets investigate' and could care less about what has been done prior. Heck they don't have the attention span to pay attention to what the findings were yesterday, let alone years ago when this was investigated and cleared prior.
So this becomes the best tool of any POTUS. Just call for, over and over again any investigation to any opponent over anything that looks wrong or swampy with the only justification needed being it looks 'wrong' or 'swampy'.
"Who is my top opponent now? Ok I want any file from the IRS, CIA, FBI, etc that may have ever been made about that person to see if we can re-investigate them".
it is a great way to rile up the most stupid of voters who will not look back and we saw it directly in the Hillary and Trump campaign. And as much as I hope Hillary never runs again I promise you if Trump or other was to raise the emails again if she did, a large percent of people would think an investigation was justifies and would defend it because they would again say 'ya but it does look wrong, so we should investigate'.
The Founders, very deliberately were trying to protect the election process from the dumbest of the voters by highlighting as the #1 and #2 impeachable offenses 'soliciting foreign investigations into a political opponent' or 2, 'using the institutions (FBI, CIA, IRS, etc) to investigate political opponents' as they could see how easy it would be for a corrupt POTUS to do so.
Trump has done both.