Actually, you can. It's entirely up to you what you do. If you cancel, just do it assertively and politely. Maybe run it by your girlfriend first in the interest of transparency and getting her buy in. Tell your gf you want to cancel because you had already said No and you meant it because you don't want to spend the money right now. Tell her you feel you were pressured into finally saying yes after you had had a couple drinks, and now that you're reflecting on it, you're upset that you were pressured into changing your answer and are going to cancel to set the boundary that No means No, this is not something I've budgeted for nor want to spend my money on right now & so you won't be going and hope that in the future if you say No to something, one, the other, or both of them well accept and respect your decision instead of pressuring you to spend money you don't want to spend.
Prepare to deal with her possibly being argumentative about it. But if it's the way you want to go with this, do it and stick to your guns and tell her you can't just spend money because your friend wants you to, you said No for a reason and your answer is No and when you say No in the future maybe they'll then realize No means No and not "maaaaybe if you keep pestering me I'll cave."
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No
for supporting trump. Because doing so is deplorable.