ORD can be a black hole for delays any given day of the week let alone when there may be ATC staffing issues. Winter, Summer, doesn't matter. All it takes is for one domino to fall then things just keep cascading. Chances are you will be just fine, although 70 mins is indeed tight. Depending on your comfort level and how bad you feel you need to be at your final destination on time, you may want to proactively take an earlier flight or seek options at another airport. No one has a crystal ball that can say for sure how things will unfold in advance.