I'll risk my few reputation points and speak to the unloved elephant in the room:
You can down-vote my answer, but you can't down-vote a candidate or party.
On what ballot have you seen "Check the party you would prefer to lose"? Even if you're one of those spiteful "third party" trolls who deny the Dems their God-given right to all non-GOP votes, you STILL aren't voting "against" anyone. (I'm not trying to pick on Dems, but I honestly have never heard a Republican lay claim to 3rd party votes.)
So the original question is illogical. I would vote "against" it remaining open since, despite the mass-hallucination of its validity, it is factually, provably not based in reality... but in the name of fighting the good fight, here's an explanation of voting:
5 + 4 = 9
Controversy aside, the sum is 9. The 5 and 4 each contribute to the sum, but neither one gets to BE the sum. That doesn't mean their values don't matter, but if 5 expects the sum to be "YOU WIN, NUMBER 5!" then 5 doesn't understand math. And if we throw in a 3? It's an actual operand, just like the rest (no, not a traitor operand). Crazy.
"Yes, but we have a highly democratic 2-choice system, so either a Republican or a Dem wins, not an average of both of them, so you're dumb."
Don't confuse an election with governance. One is an event, the other is a clusterf... a process. Despite our eagerness to gamble away all checks and balances just for that "next big win where MY guy wins and dictatorial control is a good thing!", we still have them. We also still have countless representatives of all stripes, at all levels, and they actually make decisions. It's honestly not the president calling all shots.
"So should I vote or not? What does my vote signify if my guy isn't gonna be boss?"
From the egocentric viewpoint a vote means:
"My opinion is super-valid, so I personally have the power/duty to direct the fate of humanity. If it doesn't turn out right I have failed."
The media and entrenched parties may benefit from this lie our egos tell us, but Democracy does not. Voting is not about your preferences or mine, it's about our statistical mean. That's like, not just my opinion, man, that's math. It's kinda the point of having everyone vote on the same thing, instead of just holding a vote-lottery where one person gets to decide. No vote "matters", but all votes matter. The votes of "winners" as well as "losers" will all be counted, impact discourse, etc. If you can't accept that your vote will be one among many, and might only count as such...? Well, then:
TLDR; "Math is stupid. Don't vote."
I guess that should be at the top. Sorry.