You're absolutely correct that the political affiliation, and the motivations, of The Whistleblower don't change the facts. The Trump Administration has released a transcript which essentially corroborates every one of these whistleblowers claims regarding the content of the phone call. Whatever the political beliefs and motives of The Whistleblower, this can't be changed. Since the whistleblowers claims are factually accurate, their motives, somebody, are irrelevant.
Trump is running a standard Playbook with cheat, and conservative media, has used to diffuse previous candles. The idea is to argue that these claims are all false. To buttress the idea that people are lying, they try to establish that they are politically motivated to try and see Trump and thus are not telling the truth. They've done this before, Christine blasey Ford, and with various figures in the Russia investigation. The problem now is that we have corroboration. Trump had not released the transcript then perhaps they could have made this argument that The Whistleblower is some disgruntled Democrat it is lying to Trump. These claims have often been very fishy and don't stand up to a lot of scrutiny. However, they serve the purpose of allowing conservatives a fig Leaf with which they can resolve the cognitive dissonance of wanting to support a conservative political figure but hearing disturbing allegations about them. If conservatives can make a prima facie argument that these allegations are all lies than they can dismiss them without much critical thought I'm so right back to supporting this conservative figure. But, in this case, it's impossible because Trump released the transcript. This shows that the Whistleblower is telling the truth.