Trump said some words, not the 14, but getting closer all the time--that he is a nationalist. What does that mean? Well, if he hadn't prefaced it by saying that he isn't supposed to calling himself this, then one could interpret this as Trump being an American nationalist--proud of being an American, whatever that means. But since he prefaced it as something that he should not say, well, it is easy to suggest that he was really saying he is a white nationalist even if he denied it today.
Let's step back and think about nationalism. It is generally referred to a belief or desire or a movement that seeks to have a particular nation have its own government. That, by itself, may be neither good nor bad (although I am a fan of multi-national states), but the key is this: nationalism is also about defining who is a suitable member of the nation and who is not. Much of it is not just the aspiration to have a country for the nation but who should be in that country as a citizen, who is not as suitable and how these "Others" should be treated.
And that is where Trump and his statement come in. Trump and his allies want to define the American nation very specifically--that brown folks, that those not born in the US, that transgender don't count as real/true Americans. Sarah Palin's stuff on Real America also defined American cities as not being part of Real America, but Trump as a New Yorker does not quite go there. To be clear, within the far right (and far left), there will be differences and fights over who really counts and who should be excluded, but they generally agree that the US is a white Christian (narrowly defined) nation, and those who do not fit that combination of categories are not really American.
Seems strange for Trump to be on board with this given that he has a Jewish son-in-law and Jewish grandchildren, right? Not really since Trump has long had anti-semitic views even if he thinks he can love specific ones--remember, he wanted Jewish accountants, not African-American ones, because Jews are good with money: "Black guys counting my money! I hate it. The only kind of people I want
counting my money are short guys that wear yarmulkes every day..." Trump may not be a Nazi, but he does not mind hanging around with such folks--"there are fine people on both sides."
As Trump panics about the midterms, he needs to get his base to the polls. Who are his base? Who were the key folks who helped him win the primaries? Yeah, he knows. So, this "I am a nationalist" is an appeal to those folks. Is he sincere? Well, Trump isn't sincere about much, but he is consistent about a few things, and his white supremacy (and its frequent companion--misogyny) is one of them.
So, yeah, Trump is a white nationalist, although I prefer white supremacist. His focus on the folks caravaning to the US, his mostly outdated concern about crime in inner cities, his empowering Sessions, Miller and Kelly, his attempts at a Muslim ban, and all of that is a part of a larger pattern. He is fomenting a nationalism in the US that excludes much of America, and we should find it deeply disturbing.
I do think it was a mark of desperation that Trump said what he said, but I don't expect him to stop if his party wins or if it loses the midterms. Either he will be empowered or deeply angry. It is going to get worse before it gets better. And, yes, it can get worse.