The Roman Senate, at least in Republican times, was not a legislative body. Legislation in Republican Rome was initiated by elected magistrates and voted on by citizen assemblies.

But I didn't come here to offer a feeble two line answer. While you wait for an askhistorians grade response, you might want to check out this answer by /u/Celebreth and this one by /u/xenophontheathenian, both of which go into some good detail regarding the Senate's role in Roman politics.