Will chewing tobacco instead of smoking it in the pre-hair transplant period have a bad effect on surgery?

Any nicotine intake either in form of tobacco or cigarettes, will definitely have a negative impact on your transplant results. There are more chances of graft failure and poor wound healing if you are a consumer of nicotine (current or past). 
You can discuss your case with your surgeon to have a tailored approach. You can undergo low density transplant in first session and can undergo a second session after 6-9 months to increase the density. This can prevent any complications stated above.

Nicotine in any form (smoking, vaping, chewing, patches, gum etc) causes blood vessels to contract, decreasing circulation to the skin and can cause healing problems.  If possible it's best to avoid nicotine before and after surgery, and be sure that yous surgeon knows you use nicotine as this may alter how aggressive he/she can safely be.

It is not recommended that you take any nicotine following any type of surgical procedure because it restricts the blood flow. Good blood flow is very important during the healing process and also for the success rate of the grafts. 

The use of any nicotine containing substance will cause vasoconstriction of microcirculation and restrict blood flow and affect growth and development of follicles. This says nothing about how oncological problems and the chemotherapy treatments causing alopecia totalis. Star this new year with good habits. 

The use of tobacco with any hair transplant in any manner where the tobacco is absorbed by your body, is never a good idea when you have a hair transplant