So long as there is a modern nation of Israel to support, evangelicals will not abandon it. It's that Biblical literalist thing. Nothing fickle about it.
Prior to 1948, it was not uniformly true. Various strains of belief, some clinging to "replacement theology" justified antisemitism on that basis. Some still do today, but these are not the evangelicals of whom you speak and who you disparage.
Fulfillment of prophecy is taken seriously, as is the directive to support the nation that is the fulfillment of prophecy. Then, there's the "curse" for not doing so. Not something a Biblical literalist is going to chance, taking it literally and all.
It's really not that hard to decipher, honestly. You don't even have to subscribe to the belief in order to grasp it.
And after that, what? Back to persecuting Jews?