Former Ford CEO Mark Fields said that the United Auto Workers corruption probe will make it harder for union leaders to get their rank-and-file members to sign off on any potential new labor deal with Detroit automakers.

"When you reach a tentative deal, the [companies] are relying on the union leadership to sell that to their rank-and-file," Fields said on CNBC's "Closing Bell." "So it doesn't weaken the UAW's negotiating power, but what it does do is weaken the loyalty and the trust that the rank-and-file have in the leadership."

He said General Motors has taken a hardball approach in negotiations this year. "At a minimum, it doesn't build good will with the rank-and-file, and at worst, it angers them and makes them dig their heels in," Fields continued.