Leaking portable air conditioner

It's possible but it will be creating a situation where your a/c is fighting against itself, because you'll simply be redistributing the heat inside your house, instead of putting it on the outside.

Why? Because the warm air will creep back into the room you were cooling, so the a/c will be acting like a big do-nothing machine - to a degree anyway.

Mold isn't going to be too concerning, however, because you won't be taking moisture out of the one room and putting it into the other - the moisture in the air in the warm room may increase a little (because warm air can hold more water vapor than cold air, so if moisture is there, it'll get picked up) but not to the degree it would if you were, say, venting a dryer into that room, where you'd be introducing lots of NEW moisture.

All that said - I do not recommend this idea. Better to create a vent/opening (properly, of course, but that's a different question) in the outside wall to allow the warm air to go where it is supposed to.

If your AC is seriously undersized, the compressor is probably already running 24/7 and blowing hot air into it won't make any difference. At max capacity an AC can remove a specific amount of heat from the air per hour. If you blow hotter air into the intake you will get correspondingly hotter air coming out. It's a zero-sum game.

In addition to closing off other rooms as others have suggested, here are things you might try:

• Block as much sunlight as possible from entering the rooms
• If you still have any old-fashioned incandescent bulbs, replace them. Ideally with LEDs, but compact fluorescents are cheaper up-front.
• Turn off any appliances you aren't using. If you need to run a small fan to properly circulate air that's OK, but you should be aware that the fans are generating heating and in that way they work against the AC, so it's a tradeoff. Some larger fans can use 100 watts or more of electricity, which is turned into heat. 100 watts of electric consumption generates about 350 BTUs/hour of heat.