I notice that Twitter is simply removing API access to the "deleted" tweets rather than actually DELETING them. So there's a clear agenda here, and what Twitter is doing by this course of action is tantamount to censorship. Hopefully the transparency sites will be able to whip up some sort of scraper that collects pollies' tweets and backs them up outside of Twitter's control.
Once you put it online, it SHOULD be permanent. Otherwise you're setting the stage for the Ministry of Truth on a global scale.