The simple answer is no. To deduct the mortgage interest, the payer has to be a property owner and the person obligated on the mortgage document. However, if you are in a position to benefit from the itemized deduction, you could treat it as if your partner gifted you the money and then you paid it, and you take the deduction.
If your partner is an owner of the property, then they can deduct the interest even if they are not obligated on the mortgage document, if you follow the tax laws instead of the advice in the IRS publications (the publication is not entirely accurate on this point; the law allows an owner to deduct the interest.) Your partner would check a box for "I did not get a 1098" (since it wasn't in their name). The IRS might or might not ask questions.
The most complicated situation is one where your partner is treated by the law as an owner "in fact" even if they are not an owner on paper. Also called constructive or beneficial ownership. This situation is covered in a couple of tax court cases and can go either way depending on the facts. In this case, I think it would be a losing proposition, because you have the ability to get married (which would make the deduction unquestionably proper) and choose not to. I can give a longer answer about constructive ownership if you want to push the issue but I don't think it will work in your case.
