Can you provide a pair of examples in which Snopes, Politifact, or a similar mainstream US-based fact checker used clearly different standards for what constitutes a question of fact based on the political orientation of the subject?
Choosing what statements or what formulations of statements to fact check is also a part of the bias. People just read the headline, if they read "fact check, X was not a 'convicted terrorist'", they will read that as "X was not a terrorist", the fact checkers knows this and uses that to their advantage when picking the sentence to check and the verdict that will follow.
[1] https://www.snopes.com/fact-check/cruz-wuhan-tweet/
[2] https://www.snopes.com/news/2021/07/16/lab-leak-evidence/