Learn Polish: Wierzę w tę akcję. - I believe in this campaign.

Because the verb wierzyć (to believe) specifically requires w followed by the Accusative case. You learned earlier that w + Locative means being physically inside a place, but for abstract targets like belief, transformation, or involvement (such as inwestować w or angażować się w), Polish uses w + Accusative.

I remember learning that wierzyć takes the Dative case. Why is it using a preposition here?

Adding the preposition changes the meaning. Wierzyć + Dative without a preposition means to believe what someone says. For example, wierzę ci means I believe you are telling the truth. However, wierzyć w + Accusative means to believe in the existence, value, or success of someone or something. Here, you believe the campaign is a good idea.

I often hear native speakers say w tą instead of w tę. Which one is right?

tę is the strictly correct, formal Accusative feminine singular form of the pronoun ten. However, in everyday spoken Polish, native speakers very frequently substitute it with the Instrumental form tą. At a B2 level, it is good to know that tę is what you should write in exams or formal texts, even if you hear tą used informally.

Does akcja always mean campaign, or can it translate directly to the English word action?