Divining the popularity of the Klan is not unlike divining the popularity of the NSDAP. There are myriad reasons that people joined both during their height-- political, social, economic, ideological, etc., regardless of whether they believed in the party/klan or not. The "first klan" of roughly 1866-69/71 disbanded officially after the [Ku Klux Klan act of 1871](https://njsbf.org/wp-content/uploads/2021/02/The_Ku_Klux_Klan_Trials.pdf) authorized the use of harsh penalties and military force against terrorist organizations, but the pattern of political and racial violence was not simply a product of the organization-- Another historian wrote it well, “The Ku Klux Klan was the outgrowth of peculiar conditions, social, civil and political, which prevailed at the South from 1865 to 1869. It was as much a product of those conditions as malaria is of a swamp and sun heat.” (“Ku Klux Klan: Its Origin, Growth and Disbandment”, John C. Lester, 1884).

Lester's description of the Klan there is pretty on-the-nose. Since this sort of political and social violence happened well after the Klan act of '71 (e.g. [The Chisolm Massacre](https://www.gilderlehrman.org/collection/glc09304)) had decentralized the Klan, "Kloo Kluxes" simply became the de facto term for what amounts to what amounts to White Separatists. There was no "official" KKK from 1871-1915, when the "second Klan" was formed in Georgia. The "second Klan" was largely inspired by the novel The Clansman, which romanticized the "first Klan" and would inspire Birth of a Nation. By the 1920's, the Klan was not only a White Supremacist organization, it was also a [pyramid scheme]( https://scholar.harvard.edu/files/fryer/files/hatred_and_profits_under_the_hood_of_the_ku_klux_klan.pdf?m=1360041666). It really wasn't until the Klan was essentially forced to buddy up with the Nazis in the 1930's (coupled with several high-profile scandals in the press) led to a dramatic decline in membership during the years immediately preceding WWII, and the disbandment of the second Klan in 1944 following an IRS tax levy.
With all that in mind, the Klan grew in tandem with the "great migration" of southern blacks to better opportunities in the North. Detroit forwent the burning cross in favor of electric lightbulbs, but the reaction was largely the same. The Klan itself might have been unpopular as a black eye to people that agreed with them on substance if not means, but at the end of the day the Klan was (and is) a reaction to the world around it. The Klan is a reactionary group and doesn't really have a well-defined platform historically-- as the public liberalizes, the Klan just reacts against different things. Sometimes those things garner stronger reactions from normal people that would otherwise not support them, especially if the weather's right.

While my area of study is the 1871-1915 era of the Klan, it's popularity is going to largely be dependent on the population of the given area, the number of black people living there, the economic situation, and the education of the population. It's kind of impossible to generalize someplace as big and complex as the US, but in the state of Mississippi as an example-- there is a direct correlation between the dependence of a given county on slave labor in the antebellum years, educational availability, income, and population of blacks in a given county. The coastal counties of Mississippi had relatively little Klan activity whereas the Delta region had an infamously active network of Kloo Kluxes.

As far as I'm aware, they've never represented the majority of the US as a whole, but again-- when you start digging into specific states, counties, and cities their supposed popularity might surprise you.