Oh man, I'm not sure how to condense this much context.

1. Since the days when the USA was economically reliant on slavery for land development and market growth, the US population has been split over the issue of race and ethnicity. Even before that, the USA was founded by religious conservatives fleeing the church reforms in Europe. "Freedom of Religion" was put into the constitution not to separate church and state but to protect church from state. Because of these very strong and very harmful ideologies, naturally the people split into two camps: for ethnonationalism or against.

2. The US Constitution is very old. The USA as a country is very young, but it's still one of the oldest democratic systems of government still in use today. It is very flawed: utilizing the electoral college, capping the seats in the house, each state with wildly different population getting two senators, the senate confirming judges, and worst of all "first past the post" ballots. In hindsight a lot of this is terrible for a functioning democracy, but the ethnonationalist party doesn't really like democracy anyways so it's going to take a supermajority to fix it, if you even believed the opposition party were motivated to fix it.

It's kind of like how the Weimar Republic was before the Nazis took over. There is a united hard right party and then theres the SPD. You COULD split the SPD's influence into farther left and communist parties, but then if they don't individually have enough seats they fail to form a government the Nazis have opportunity to become majority in the face of continued inaction from the government.