Just out of curiousity, have you looked at refaring the ticket to go thru ATL instead of MSP? If you can change the ticket for $0 and route via ATL, I would recommend that. If you can't or don't want to, then as noted in another post, monitor MSP for weather and check delta.com for potential weather advisories and change waivers. If there are no weather issues and your flight departs TUS on-time, you should be more than fine with 55 minutes in MSP, albeit with not much loiter time depending on how far you have to go from your inbound gate to the outbound gate.