I've bought this game a year ago or so and just now I had the time to play it. There is a problem, though. Is there a mod, a patch...even a virus...that allows me to save during a mission in the campaign mode? My necessity it's not to cheat or to restart a mission, it's just that sometimes I have to go out for some reason and I lost a mission just because I can't save. It's really annoying.
You simply cannot as it would be easy to abuse the campaign system with that. It is just a game design choice the designers made.