The SPLC, which has a vested interest in making the numbers of hate groups as large as possible, especially white nationalism, puts their numbers at less than 10,000 nationwide. In a country of approximately 300 million people, that makes them 0.0033 % of the total public, hardly enough to be anyone's base of support.
The SPLC makes fine-grained but not politically meaningful distinctions between "white nationalists", "white supremacists", "racist skinheads", the KKK, "Neo-Nazi's" and many other white racists groups that share largely the same views and policy positions in any way relevant to this discussion. There are over 100 formal KKK chapters in the US and about 500 identifiable organized groups that fall into these various sub-categories.
In addition, estimates of people that belong to such groups are based upon where there is evidence of an association to a particular group. That guarantees an under-estimate of people who hold such views by a factor of somewhere between 10 and 100.
It not only excludes people who personal identify with a group but are careful not to do so in any public fashion, but also excludes the vast majority of people with such views who do not affiliate it with any formal organization that specialized in promoting such views.
And as I explain in more detail below, it is an undeniable fact, one which Breitbart itself openly acknowledged, that its fanbase and the entire alt-right movement is rooted in the "intellectual arguments" of historical white supremacists and the originator of the term is an unabashed white supremacist. Their numbers alone are in the millions, regardless of whether they formerly belong to a recognized white supremacist group (beyond Breitbart itself).
During the election, two people came forward to speak for that demographic. One of them expressed support for Trump (but didn't go all the way to an endorsement) and one of them expressed support for Hillary (but didn't go all the way to an endorsement).
Utter nonsense.
A white nationalists was one of Trump's pledged delegates in the CA primary.
Trump had a Q&A with his fanboys on Reddit forum called The_Donald which regular hosts discussions where most subscribers are white nationalists that endorse ideas like calling Black Lives Matter protests as "chimp outs", the evils race mixing, Nazis' high IQs, and the excessive Jewish influence in America.
Trump's appointee to the DHS's "immigration" sector, Julie Kirchner, was an 8 year Exec Direction of Federation for American Immigration Reform. That organization is one the most powerful anti-immigrant lobbying groups with strong ties to the GOP, and was founded by a bigot who ranted about the doom of the "Latin onslaught" and that "One of my prime concerns is about the decline of folks who look like you and me.” "For European-American society and culture to persist requires a European-American majority, and a clear one at that.”
Their annual conference is rant-fest circle jerk that includes extremist right-wing talk show host of the sort that are fanboys of the criminally racist Sherrif Joe. They were funded by the white supremacist "think tank" The Pioneer Fund whose long time director Rushton who often published his overtly racist pseudo-science promoting the genetic inferiority of blacks in the
American Renaissance rag and spoke at their conference, which is a well known white supremacist organization that even describes itself as "race-realist".
Trump was fully on board with the "Obama is a Muslim" and birther nonsense that only white supremacists would dream up or care about (which includes the nutcase Alex Jones and his followers, who are almost all diehard fanboys of Trump).
Oh, and almost all who qualify or identify as "alt-right" fit into at least one of these white racist subgroups.
By it's own admission Breitbart has admitted that the alt-right the comprises their fanbase are "mostly white, mostly male middle-American radicals, who are unapologetically embracing a new identity politics that prioritises the interests of their own demographic." AKA white nationalism.
The same article points out that the main difference between them and the type of skinheads on the street is merely that the alt-right is more "intellectual", turning to "thinkers" like Spengler, Menken, Evola, and Samuel Francis who (despite the article not noting it) are/were all undeniably white supremacists.
The "alt-right" meme took off from the website alternativeright.com blog founded by another undeniable overt white supremacist Richard Spencer in 2010, who was inspired by a speech given 2 years prior the to annual meeting of the HL Mencken Club (aka a room filled with white supremacists). This founder and hero of the alt-right organized a rally held in a Federal building in DC 2 weeks after Trump's election. After 11 hours of countless speakers praising Trump and speaking like they just rose to power, Spencer concluded with a speech filled with quotes directly from Nazi propaganda that he spoke in the original German talking about the “children of the sun,” as race of conquerors who under Trump, were “awakening to their own identity.” The crowd responded with Nazi saluted and "Heil".
Which of course means that Trumps main advisor for the last couple years, Steve Bannon, and all the voices on Breitbart are supporters of Spencer and this alt-right movement rooted in white supremacist pseudo-intellectualism, since they have explicitly and proudly declared their site to be "the platform for the alt-right" and proudly proclaimed their "dangerously bright" audience as being fueled by the who's who of white supremacist "intellectuals" of the last century.
Breitbart alone gets 10,000,000 visitors every month and is almost unwaveringly pro-Trump and was rabidly anti-hillary, so to pretend these racist shitbags are not a huge % of Trumps base and overwhelming more GOP than DEM requires the pinnacle of intellectual dishonestly (which is coincidentally another core attribute of the alt-right).
Bottom line is that without people who accept in large part white supremacist/nationalist assumptions and ideology, Trump and the GOP would have no chance of winning any election at the national, and very few at the State level. They all know this, and know that is partly a cause why and partly a product of the fact that the GOP consciously chose to court the votes of white racists with their "Southern Strategy", which is why and when one of the most blatant racists to a Congressional seat, Strom Thurmond, switched parties in 1965. Coicindentally, his heir to the most blatant racist prize goes to Jeff Sessions, Trumps best bud in the Senate.