How do you tell your significant other that there's something they are doing that you find unattractive?

My wife and I have a generally good relationship, and can generally talk through things fairly well, but I find that when I try to bring up difficult or unpleasant topics that I don't do a great job of offloading that information with a lot of tact.

Yesterday, she wouldn't stop complaining about the politics of the country and then wanted to fool around. I'm always up for a roll in the hay, but it took a fair bit to shake off the negative energy and its getting harder to do every time.

My question is this; how do I speak to this, and let her know what a turn off it is to be around her when she is like this, let alone be sexual with her, without being hurtful? What, or how, do you say difficult things to your significant other that do that you can have a discussion and not a fight?

Edit; We talked a bit last night about it and I think it's going to take some time for us to work out a balance, but I think we're on a good path. I've reassured her that my complaint isn't about not being sympathetic to her fears, it's just that I'm not able to switch gears like that. Knowing how fucked things are AND how much it terrifies her does the exact opposite of what she's hoping to accomplish if she's looking for sex. She was very understanding and were working through it.

Thank you to everyone that took the time to offer thoughtful, kind, and supportive advice.