My interpretation is that Americans don't actually care about unions despite what they might say. They like unions only so much as they help boost incomes, but they don't actually have any solidarity as workers. In our society of heavy individualism, workers are concerned only for their personal benefits and not the union as a group. Instead, they tend to identify on racial and gender lines more. So they're ok with losing their union if the next president promises higher incomes and fewer other racial groups, like immigrants.