Maybe I am missing something here. I am all for Democrats openly resisting denouncing Trump's presidency, but the ceaseless focus on calling him "illegitimate" is just inane.
It's like the "not my president" BS. Yes he is your president (or will be in a few days), just as Putin would be your president if you were Russian. Refusing to acknowledge the fact means you won't be able to fight him as effectively.
Given the way the Trump and his future administration have signaled their intention to bend over for him, one could argue that Putin effectively will be our president.