The fact that Trump lied about being in Moscow indicates that whatever he was doing there - he does not want the public or investigators to know about.
It's not illegal in and of itself, but it is certainly a smoking gun.
Or, that, as a private citizen, it’s simply none of your damn business.
Once someone runs for public office everything about them becomes everyone's business.
The Republicans have certainly always believed that.
For political purposes not for criminal prosecution.
All these issues are fair game during the campaign.
Voters can use them to determine who they are going to vote for.
After the election,moot.