luta,
You need to be a US citizen to apply for your parent's Green Card. If you are close to becoming a citizen, I suggest filing for a visa extension for your parents. That extension will take a month or so to get approved or denied. If you already have a citizenship interview lined up, it would make sense for your parents to stay in the US a little longer on a visa extension. Once you are a citizen, you can apply for their green card.
If it will take you a while to become a citizen, then your parents will have to go back, otherwise they will be in the US illegally.