Honestly, they might. A lone wolf might attempt it, but it will be worse if Trump wins.
If Trump wins the presidency, he will kill or jail anyone he doesn't like. The Republicans plan to take control of the federal government without congressional support, and Trump openly tells us his intentions. Even if Romney didn't say this, Trump would still target him, so of course he would vote against him.
It's not a joke. Trump became more dangerous and more driven to become a dictator by losing in 2020. He was always going to try to overturn the constitution to become president for life by 2024, but now there are no neoliberal conservatives to hold him back. It's total fascism time for Republicans.
It's legal plausible deniability. They tell the lone wolves what to do with coded language on social media, but don't directly communicate with individual terrorists.