You can submit once the old poll has closed and the new submit thread has opened. You have to at least have 500 post to submit. If it makes it in the top 30, you'll be able to vote in the next round.

Keeping the thread active.

You can't go 7 days without posting or the campaign becomes inactive. If that happens and the Campaign is in voting round, or if they make it to the next round of voting they become disqualified and the next in line gets the spot. To keep the thread active if no one has posted, you can double post on the 7th day to keep the thread going. Just don't do it too many times. I don't think it'll count then.