Is it rude to ask someone about how they voted? Nope, no way. Not if you sandwich it between "How much weight have you gained recently?" and "Do you think you're single because of your personality or your looks?" It's all in how you frame the question, you see!
But truly, asking someone how they voted is — by and large — not considered polite conversation, and in some places it's not even legal. (We'll get to that later.) While some people may not mind the question, others consider it downright offensive, along the lines of asking someone's salary or weight or how much they paid for their house. Or maybe the person voted for the party you can't stand but doesn't feel like getting into a political argument with you ("Why'd you support him? Are you nuts?") So, why risk asking?
However, that doesn't necessarily mean any mention of politics is off limits; in fact, trying to dismiss a direct political question might mean engaging in a more broad discussion of the political process. If someone has asked you who you voted for, the estimable Miss Manners suggests you turn the question into a conversation about the nature of the secret ballot and its changing importance through history [source: Martin].
Now, no offense to Miss Manners (which would merit jail time), but not everyone is capable of sidestepping a volatile personal discussion with a more general political discussion. If you've been asked about your political inclinations or choices, it's probably best to give a cheerful dismissal and move on. ("Oh, I'm sick to death of talking politics, unless it's 'House of Cards.' Did you watch it?")
You also should be aware that in many states in the U.S., you're specifically not allowed to do any campaigning in or near a polling area [source: National Conference of State Legislatures]. That doesn't mean you'll be thrown in the gulag if you ask who someone voted for (or volunteer the information about yourself), but it's something to be aware of if you're hanging out at polling places and trying to persuade folks to vote for the countywide cat-leash law.
But Britain is a different story. Talking about the election in a polling place — period — is off limits. If you're so much as speaking with your spouse about whether to vote for the city councilman who wants to ban recycling or the city councilwoman who wants to ban trash collection, you'll be asked to go outside [source: BBC]. Also, you can't vote if you're a major royal. So if you run into Prince William at the grocery store, don't ask him who he voted for. It's doubly rude.