There is a difference between being discreet and being invisible. In this case I think that discreet means being polite and unobtrusive as possible. You don't want to be standing in front of some VIP in order to get the shot, but standing up and walking around is probably fine. As Bitter said, many of these are questions best posed to the person who hired you.
The other question I have is, what is the intended use of these photos? I assume that it is for publicity in some form - both for the politician and his guests. In that case, taking pictures is only part of the package, you'll likely want/need to be able to identify the guests. The most relevant example I can think of off the top of my head is that, quite often on the main page, Boston.com has a photo section in which their photographer posts pictures of party guests at events along the lines of what you'll be shooting (of course there's no link for me to copy right now). These are posed, small group shots, and he identifies each person in them ("Joe Schmo and Jane Doe, both of SomeTown, MA"). It's an ego stroke for the guests (i.e., potential campaign contributors). Since this is, I assume, a private event there may be someone in the politician's camp who can identify all the guests, otherwise it might be up to you to collect this info as you shoot them. Again, it goes back to clarifying what the expectations are from the client's side.