First: I really don't think a vote for President Trump is a vote for conservative principles. The biggest conservative principles is a love of liberty/individualism/fear of autocracy and a preference for historic institutions and approaches that have a track record of working well. Trump has proven time and again he's not a great supporter of either of those.
For the record- these aren't bad things. I actually agree with them to a point, though I think they also don't tell the whole story.
But my point- people like Gov. Christie (and a good share of my family!) who think these principles should be the main guide for our government may have a candidate but they don't have an advocate for their worldview. A vote for Trump isn't a vote for conservatism. And more than that, since Trump is a danger not just to America but conservative principles (let's leave the racism etc. baked in to the Southern Strategy to the side for now)... conservatives pretty much have to vote for not-Trump. They can't sit this one out and they can't make a protest vote for a third party.
In our two-party system with the way this election rolled out, not-Trump means Joe Biden.
But this isn't actually the whole of the story, because progressives are just as threatened by Trump. If conservatism is about personal liberty, progressivism is about giving people the positive ability to make those choices. (You don't have the freedom to choose where you can live if you can only afford the rent in very limited neighborhoods.) And if conservatism is about respecting tried-and-true institutions, progressivism is about recognizing the way those institutions haven't worked for some people and using data, science, and logic to improve on them without being fettered by the past. And while Donald Trump might be breaking those conservative principles he's not exactly leaning into progressive ones either!
So progressives aren't able to make a choice for who they think best represents progressivism; this time it had to be the person who could beat Trump, which is a very different question. And they should be working with conservatives opposed to Trump (and many are; see the conservative/Republican voices given speaking time at the recent DNC onvention); but they shouldn't take this alliance to mean those voices are suddenly progressivism. They should be given a seat at the table, but not at the progressive table, at the let's-get-this-trashfire-out-of-office-p
Put another way, just like conservatives don't have an advocate in this election, progressives don't either. I don't mean because Biden isn't a good advocate for progressivism. It's because the way the discussion has to be framed, it's not about progressive-versus-conservative, it has to be fought along different lines entirely. Even if Biden wins (and please God), it's only a progressive victory in the sense that a Trump victory is a conservative victory, because he had his origins in the progressive wing of American politics and the majority of his supporters hail from that camp. But really, the battle is and always was about a different issue entirely.
What that means is that this whole election has already been stolen from all of us, and I don't mean from the attacks on the post office. Trump's mere presene and the way he totally coopted the RNC and conservativism means progressives don't get to have a debate over where we should draw the line between positive and negative liberty (freedom to and freedom from), or whether institutions like the police and courts and the way we pay for health insurance and affordable housing and education and all that are actually working well. Progressives lost out on this essential conversation, too. Because that's what an election is, in addition to all the other nonsense: it's a conversation where we discuss our ideas and values, then we each get to say who we most agree with by voting and everyone gets an equal voice for at least that one moment.
President Trump's stolen that from us. And honestly? I'm pretty pissed about that. (Other stuff too! But that as well.)