I'm not sure if we'll ever see brown shirts. But it won't matter. You don't have to have brown shirts to live in an authoritarian regime anymore. With social media and disinformation campaigns combined with crony capitalism, and the way we elect the senate and electoral college, Trump and the Republicans could turn off 60-70 percent of the country yet still control most of the government.
#1 They bribe the rich and corporations with handouts.
#2 States outside of the coasts are becoming increasingly homogeneous, older, and red. Smaller and redder states count just as much for the senate as states like California. So Wyoming essentially negates California's representation in the Senate. This permits Republicans to perpetually control one half of Congress and appoint judges. Support for labor, women, and minorities already is being cut. Ask a LGBT person if they feel Trump's authoritarianism over them. Or the latino community. Ask women once Kavanaugh guts Roe v Wade if they still feel free.
#3 The disinformation campaign prevents a tipping point of the populace from ever turning on "their man." As a result, governing becomes harder and holding people like Trump accountable becomes impossible. Putin, Erodogan, and Trump have mastered this. As long as 30 percent of the populace still think that Hillary is the one who colluded, MAGA is awesome, and Democrats actually framed Cesar with False Flag nonsense, it's impossible to hold Trump and his cronies accountable.