Tell them that you are extra concerned about security because you are running a political site, and ask them what their record is like. If possible, talk to other organisations that already use that service provider. Use anti-virus software on your campaign machines, and ensure it is kept up to date. Be wary of all email attachments. Never open files attached to emails from strangers, and be wary of attachments from friends and colleagues as well.
Many viruses copy themselves to email addresses of friends.
Don't open any attachment if you have not got a positive reason to think that it is legitimate. Always scan floppy disks and CDs before using them, even if they come from reputable sources. Back-up all your information regularly. Whatever problems may occur, the only thing that matters is that you have a complete, recent copy of all your data. Restrict access to your office's computers, and particularly your web server passwords, by temporary workers.