When a person or group cannot win an argument based on facts and logic, they often resort to claim that other people or groups do the same thing. I once tried to defend myself for a speeding ticket with the claim that I was just trying to keep up with traffic—everybody was speeding. What do you think the magistrate's response was?
The fact that other people commit robbery and murder is no argument that we should hold no one responsible for these crimes. Speeding, robbery, and murder violate the law no matter how many people escape punishment for breaking the law. Cases of these infractions are dealt with on a case-by-case basis for good reason.
Republicans resort to the same "argument" when criticized for their misbehavior. How many times have I heard, "Both parties do it"? Republican obstructionism—both parties do it. In fact, all the media corporations reported it as "gridlock", implying equal responsibility on both sides without having to say "both sides do it".
Republican fake news—both parties do it. Republican dirty tricks—both parties do it. Trump campaign's dozens of contacts with our adversary, Russia—Hillary did it, too. Even had Hillary or members of her campaign contacted the Russians via Ukraine as the right-wing press is reporting, it is still wrong for Republicans to have done so. Identical bad actions by Republicans and Democrats do not in principle exculpate either party.
Now the president has raised this "defense" for the white supremacists who held the recent rally in Charlottesville, Virginia, and the violence that resulted from it. He claimed that the people who came to protest white supremacism at that rally were as responsible for that violence as the racists themselves. Even if the rally protesters did help create the violence, that fact would not exculpate the white supremacists.
Of course, we know now the president misrepresented the facts. The racists brought pipes, baseball bats, clubs, guns, and shields to the rally, so they did come with the intention of creating violence. The violent tiki torch parade the night before the rally was a chilling reminder of the night marches of the Nazis with torches and Kristallnacht to those of us old enough to remember.
Those who opposed the white supremacists came unarmed, so the claim that both sides were responsible for the violence is patently false. Violence is a recruiting tool for these racist reactionary organizations. I was born and raised in the South in the 1940s and 1950s; I know.
So, saying someone else is just as bad, does not justify bad behavior by anyone else. Two wrongs do not make a right. The same principle applies for political parties. The claim that both parties do it, even if true, does not justify bad behavior in either party. If the claim is a lie, made to excuse bad behavior, it is doubly bad.