Donald Trump’s allies are already preparing a “holy war” against pop superstar Taylor Swift if she endorses President Joe Biden. Sources familiar with the matter told Rolling Stone the former president’s loyalists and other allies assume the singer-songwriter will eventually endorse Biden, as she did in 2020, and that likely move infuriates Trump and has already prompted MAGA culture warriors to whip up conspiracy theories about Swift and her NFL star boyfriend Travis Kelce. “Behind the scenes, Trump has reacted to the possibility of Biden and Swift teaming up against him this year not with al…