So let's take stock of the excuses so far, and how they hold up on an effectiveness scale, 0 for no effect, 10 for defeats the charge.
1. The Dems have hated Trump since the beginning and have been looking for a way to get rid of him:
This is true, but it is irrelevant. The Dems' motivation has nothing to do with the objective merits of the criticism of Trump's actions. This matters only to the base, which was going nowhere anyway.
Effectiveness: 2
2. Trump is right to attack corruption:
This is true. But the excuse does not apply here because the evidence is overwhelming that this was not about some general, superseding power to combat crime, it was about using the office to seek foreign assistance in the election. No one really believes this was just Trump fighting crime. I doubt even his base really buys into that one.
Effectiveness: 0
3. Everybody does it and I can cite examples of the Clintons seeking at least foreign influence or money in a US campaign:
This is a weak comparison because the source of this claim, which is really quite murky and undefined, is the Clinton Foundation. Thing is, that did not occur whilst a sitting president. And it was not direct, as was Trump's calls for interference. It always involves some hard-to-understand deal with five moving parts. It works well for the base because they are already inclined to believe it. And guys like Jordan will be all over it come hearing time because he knows it is what Trump and the base want to hear. But it is so substantively different that the excuse is easily defeated.
Effectiveness: 3
4. There is no proof of a quid pro quo on the military aid.
This argument fails at two levels. First, the timing and other circumstances make claim of lack of a quid pro quo pretty dumb. Remember, the issue is not whether it was express. Put yourself in the Ukrainian President's shoes. First the aid is inexplicably held up. Then when you bring up the issue of buying more missiles, the first thing POTUS says is do us a solid, and its campaign related. Plus we have not seen everything else, yet. This argument is much more likely to get worse, rather than better, moving forward. And indeed even without an express quid pro quo, Trump having asked for assistance in such fashion is the real wrong, in and of itself.
Effectiveness: 1
5. Trump really did not understand that there was anything wrong with doing this, so he deserves a pass given his success in other areas.
Objectively this is a poor argument. The phone call to the Ukraine came on the heels of the Mueller report. It is difficult to see how Trump would not have understood that asking a foreign government to investigate your main political rival is unacceptable behavior. If he did think that, it suggests his ego is so enormous, and his intelligence so low, that he should not get this pass. Plus, the fact there was an effort to make it harder for people to see that transcript, even if not at Trump's personal direction, also suggests an awareness that Trump's overture to Ukraine was wrong.
Effectiveness: 1
In short, none of the themes offered so far as excuses and arguments for Trump's behavior is legitimate. At best, they will have low level effect with people who would never have turned on him, anyway. I know you supporters don't want to hear it, but it genuinely seems like the fight they are putting up now just makes this worse, not better.