Can a divorce decree be modified on who claims the children on their taxes?

Divorce decree allows my ex to claim 1 of 2 children. At the time of divorce he was exercising visitation, paying child support, we split holidays and summers. Now he has opted to no longer have contact with the children but still pays child support. I am struggling to fully support the kids now that he no longer takes them for visitation and no longer helps with extra curricular activities. Can I ask for a modification so I can claim both kids since they live with me year round with no visitation. My lawyer at the time of divorce recommended that I allow him to claim one child since he paid child support and had visitation at which time he cared for their needs. He never had 50/50, just every other week and two times a week for dinner and holidays school breaks. I thought it was fair then since he helped other than child support, now he does only child support. He makes significantly more than me a year around $50, 000 more a year.