If you have a credit card with a $25,000 limit, that limit counts against your total even if you are not using it. For example, if it is determined that you can sustain mortgage debt of a maximum of $400,000 at current interest rates, you will not qualify for that amount because you also have an open credit card with an available balance of $25,000 at a significantly higher interest rate.
EDIT: You can only decrease what you owe on a loan but a credit card is an open line of credit that you can max out at any time. Because of this, the entire credit line counts against you when evaluating your debt.