Q. So, a nonrefundable credit would still offset any remaining amount due?
A. Yes. It would reduce your calculated tax.
Using an example: at an AGI of $41,000, your calculated tax would be $1510. Your savers credit would be $1510 (not the full $2000 calculated on the $4000 contributions). So your net calculated tax is 0. All your spouse's withholding would be refunded.
