The ruling party is corrupt — but the opposition is just the same.
Literal
Ruling-party [topic-は] corrupt-[is]-ている, however opposition-party [even-だって] same-like thing is.
腐敗する ('to become corrupt / rot') is the verb; ~ている makes it the state of being corrupt. だって is a casual/emphatic particle meaning 'too / even / also.' 同じようなもの ('the same kind of thing') dismissively equates the two. A cynical political take.