Can a landlord raise the rent even though you have a written lease?

A lease is a contract; both parties are bound by its terms. This is one of the THE most settled prospects in the law--there is simply no doubt that a landlord may not raise rent in the middle of a lease, unless there is a clause in the lease itself allowing it (e.g. if there's a clause allowing the landlord to pass on utility or insurance increases, and those costs go up). You do not have to pay additional rent and the landlord tries to sue you to force you to (or to bring an eviction action) because you refuse to pay, you will win and could probably countersue for damages as well.

Of course, the landlord may raise rent as much as she likes when the lease ends, but that's a different story.