Learn Polish: Ta partia wygrała wybory. - This party won the elections.

In Polish, the singular noun wybór means a choice. When talking about a political election, Polish always uses the plural form wybory, which literally translates to choices.

What grammatical case is wybory here?

It is in the accusative case because it is the direct object of the verb wygrała (won). For plural nouns that do not refer to a group containing men, the accusative form looks exactly the same as the nominative (dictionary) form.

Is wygrała perfective or imperfective?

It is the perfective verb wygrać, used here because winning the election is a single, completed event. The imperfective counterpart is wygrywać, which you would use to say they were in the process of winning or that they win frequently.

Can I use partia to talk about a birthday party?

No, partia is a false friend in that context. It means a political party, a game (like a match of chess), or a batch of goods. If you want to talk about a fun celebration or a party with friends, you should use impreza or przyjęcie.