I just got off the phone with Delta....Back in December, they revised the scheduled departure/arrival of the second leg of my return flight (MSP to GRR) - departure was 1hr 42min later than original, arrival was 1hr 33min later than original.
I found in their contract of carriage Rule 19 that says, "...If there is a flight cancellation, diversion, delay of greater than 90 minutes, or that will cause a passenger to miss connections, Delta will (at passenger’s request) cancel the remaining ticket and refund the unused portion of the ticket and unused ancillary fees in the original form of payment."
I had to go back and forth with the agent for several minutes because she kept looking only at the first leg of each of my flights which only changed by a few minutes...finally got her to agree that this leg's schedule change was more than 90 minutes, so she agreed to a refund. She said it could take "3 to 21 days" to process.