Look at other political websites to get an idea of what they're selling. Typically they've done the research for you to see what's good to sell to that demographic. If they have their own mailing list -- sign up and you will probably get a newsletter trying to sell you stuff.
I was monitoring donald trumps twitter last month and people were going crazy spamming personalized mugs with trump meme pictures on it. Something like $22 a mug. Stickers, shirts, mugs, flags, hats all seem to be popular for that sort of political support stuff.