I would argue there is some nuance to this - justifying your favorite politician(s) behavior by claiming the other side does it as well is an excuse you'd expec

I would argue there is some nuance to this - justifying your favorite politician(s) behavior by claiming the other side does it as well is an excuse you'd expect from a child. However, when you claim that your own party is the bastion of righteousness and the only possible solution because the other party does bad things that you do as well, it undermines your argument.