If I run a home business and a portion of the home is used as an office for the business can I deduct a percentage of the mortgage?
You can claim a deduction only if all of your business is conducted from the home, i.e. your home is your principal place of business - not just if you work from home sometimes.
You cannot attempt to deduct your entire mortgage simply because you run your business out of your home.
The portion of your mortgage and other related & allowable home expense deductions has to be pro-rated to be equal to or less than the portion of your home you use for business. Simply put, if your business is operated out of a 120 sq-ft self-contained space, and your home's total square-footage is 2400 sq-ft, you can deduct 5% of your expenses (120/2,400 = 0.05).
Hope this helps!