I have my own issues with right-wing libertarianism in the US, but I'll do my best to steelman it in creating a world where it thrives. Here goes: William McKinley survives his 1901 assassination, and Theodore Roosevelt never becomes President leading to a more liberal/moderate progressive era. Frustrated at a lack of tangible progressive gains, leftist splinter groups radicalize and commit terrorist attacks, leading to a much harsher First Red Scare than in OTL. At the same time, a staunch isolationist populist is elected president at the onset of TTL's World War 1, leading to no 16th or 17th Constitutional Amendments (Income Tax and Popular Election of Senators, respectively). Horrified by World War 1 and the Russian Revolution or an analogue abroad, the works of Herbert Spencer, Max Stirner, William Graham Sumner, Ragnar Redbeard, Ayn Rand, and Ludwig Von Mises gain newfound appreciation among the ruling classes, who create astroturf organizations with chapters in myriad American communities to disseminate these philosophies to the rabble. Basically OTL's Tea Party Movement, but in the 1910s rather than the 2010s. TTL's Great Depression is even more brutal due to lack of Progressive-Era regulations, but America pulls up her bootstraps and chugs on through. The European Theater of TTL's WW2 is either butterflied away, or is a more of a Red Alert-style confrontation of "The West" with a more hawkish, Trotsky-headed Soviet Union, no doubt increasing anti-socialist sentiments even further across the world. Women's Liberation still happens but takes on a more individualist bent, and is primarily pushed by liberals rather than leftists, in the vein of Ayn Rand, or Edward Bernays' rebranding of cigarettes as "Freedom Torches" for women. Racial segregation very slowly fades away as individualist philosophies take hold over ethnic tribalism, though moreso by social taboo than actual de jure Civil Rights. After a mid-war coup by Stalin, the USSR unleashes an antisemitic purge akin to OTL's Holocaust and practically collapses in the 1950s, becoming as reviled as OTL's Nazi Germany for ages to come. A younger president Ronald Reagan declares this allied victory in the East-West struggle (TTL's mishmash of WW2/The Cold War) "The End of History" - the final triumph of capitalist liberty over socialist tyranny. I took inspiration from Reds!: A Revolutionary Timeline and Twilight of the Red Tsar, in particular.