What does Donald Trump’s “go-to argument” mean?
Solution 1:
According to Wiktionary (I think it has the best definition), the adjective go-to means:
Desired; desirable; of choice: 'Cheesecake is my go-to food whenever I feel down.'
Reliable; likely to perform in difficult circumstances: 'Smith is your go-to person if you want lasting results'.
Your example sentence could be rephrased to:
... his typical arguments on which he always relies or that we could always expect him to make whenever he find someone (talk show hosts, journalist, pundits and political consultants) to attack for why someone is a loser, a dope or a dummy is that he or she has made erroneous predictions...
Those arguments are typical of Donald Trump and he uses them so frequently that they are arguments of choice or preferred above others by Donald Trump.
[Wiktionary]
Solution 2:
I'll use your dictionary definition as the reference.
"Go-to argument" means the argument someone 'goes to' automatically, akin to a knee jerk reaction, although not necessarily with the emotional overtones.
When someone responds with the same argument each time a given scenario is posed, that argument is known as his or her go-to argument.
Solution 3:
In this context, the closest equivalent for go-to will be "default" or maybe "standard". You might also use "fallback". The choice that is the most reliable, or that involves the least amount of thinking, or risk. Sometimes it also means the "lazy" option.
Maybe you're familiar with the phrase "No one ever got fired for buying IBM". In that case, IBM was the go-to option.
"Go to" doesn't by itself have a positive or negative meaning. It can be used with a positive, neutral, or negative meaning, depending on the context. In the Trump example of the question it was used with a negative meaning, while the Cambridge examples are all positive.
Solution 4:
Usually, "go-to" (noun) suggests the standard option (on the individual level) in that context. Note that there is no implication that the user of "go-to" believes that it is the best or most trustworthy option, rather it is their automatic response when no other option takes priority. For instance, when I want food delivered to me, my go-to meal delivery service is Pizza Hut, despite the fact that I no longer enjoy their pizza. It's what I've done for years and is now the default meal delivery service I will order, unless I have something better in mind.
In this case, I interpret the situation as the authors discussing Trump's default argument (the one he uses when he doesn't have something better prepared) or his go-to argument.