I think mainly identity politics. When their team does or claims something that conflicts with their core values, they just dismiss it or rationalize it and don't think about it. My FiL said Trump will never roll back RvW but here we are. To his credit, he hasn't voted since. He'd never vote Dem, but he won't vote for Trump either. Maybe rolling back vaccines will be a watershed moment for your friends, if it isn't too late. Maybe not.
When you're talking to them, it might be helpful to ask open ended questions instead of directly challenging them.
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"I know you've always cared about conservation. How do you square that with Trump's environmental rollbacks?"
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"What makes you confident RFK Jr. won't follow through on his anti-vax stance?"
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"How do you feel about legal refugees being turned away? Do you think there's a better way to handle it?"
And then just listen and ask for clarification when they vaguely hand wave things. This approach keeps the conversation open-ended and forces them to engage with their own reasoning rather than shutting down in defensiveness. There's no guarantee of reaching them. They might get irritated when confronted with these contradictions. They might just shut down and dismiss the question but maybe it'll start a process of introspection.
There's probably going to be a lot of "I told you so" fuel coming soon. Try to avoid that among friends. Save that for cathartic Lemmy posts. Good luck.