I’m flying SBN-ORD-LAX in a couple of months on a Sunday afternoon. When I booked the flights, I had a 70 minute layover at O’Hare. My flight from SBN to ORD is leaving later and my flight from ORD to LAX is leaving earlier. So I now have a 45 minute layover. I haven’t flown through ORD in years. Is 45 minutes reasonable?
Minimum connection time on that itinerary is likely 0h40m, so you're just five minutes above that. In the very likely chance that SBD-ORD is operated by the CRJ-200 or ERJ-145, you will need to valet-check your carry-on bag, and you will likely need to change terminals from Terminal 2 to Terminal 1. Depending upon your gate assignments and walking speed, it may take up to 20 minutes to navigate from the furthest reaches of the E/F gates in Terminal 2 to the furthest C gates in Terminal 1, and you're subject to being offloaded if you arrive at the connecting gate less than 15 minutes prior to departure.
On the other hand, SBN-ORD takes about 25 minutes in the air and is usually blocked for about an hour -- so if there's no ground traffic at O'Hare, you may end up with plenty of time. However, it's definitely an aggressive connection.
Although the flight time may not have changed enough (30m) to trigger the schedule change policy officially, there is a subjective rule that allows agents to offer you a free change if your connection becomes "significantly" tighter. If you're nervous, and if UA has another set of flights that would work for you, it's worth a call.