CanI legally withhold my rent?

You can claim a breach of the "warranty of habitability" (this is a guarantee that is implied in every residential lease). Accordingly, a tenant must be given a safe and sanitary premises in which to live. Since you are not being given such a premises your landlord is in breach of your lease. You have several options for such a breach: you can terminate your lease; make the repair yourself and the deduct the cost from the rent, or (as you want to do) withhold rent until the repairs are made.

So if you want to withhold you rent under the circumstances you can. However, you can't simply stop paying. That would leave you in breach without establishing your right to do so. This will open you up to monetary penalties. What you'll need to do is to go down to housing court (or the court where landlord-tenant matters are heard in your area) and start an action based upon the breach of the warranty of habitability. The court may require you to pay your rent into an escrow fund. It will then allow the landlord to take care of the problem or try to at least fix it (you may be relocated for the time it takes them to do so).

At this point, you need to consult directly with an attorney in your area who specializes in landlord-tenant matters. At the very least, you can contact a tenants' right advocacy group. Again, if you attempt any of the aforementioned legal remedies you must be certain of your rights under applicable state law. The consequence of wrongfully using these self-help measures could cost you.