It's not an either/or thing. But let's untangle "socioeconomic", which I think in this case might be obscuring insight into our predicament.

After the election, there was an immediate, premature, consensus that what motivated Trump's knuckle-draggers was "economic anxiety", the belief that they were all people who'd seen their jobs go away and their incomes drop.

But more thoughtful study of the issue, as reported recently in multiple studies (including Status threat, not economic hardship, explains the 2016 presidential vote in PNAS), concludes that it isn't money that motivates them, it's fear of losing status as a white person. That is, the foundational narrative is the one about whites being somehow a threatened minority in an increasingly brown world.

You're also not wrong about ethnonationalism being "a factor that is deliberately seeded and aggravated". But I don't think it's something that was implanted whole and complete, rather, demagogues like Trump skillfully tease out the existing resentments and exacerbate them, and helpfully identify the villain who must be exterminated.

Trumpers are the eternal useful idiots, whose natural character flaws make them the perfect tool for rightwing authoritarians.