If everything is on-time, 50 minutes at DEN is more than fine. Even if your inbound TPA-DEN is delayed 20-30 minutes, if you sit near the front and know exactly which gate you're going to, you should be fine. Once you land at DEN, check the UA app on your phone for the DEN-MFR gate, and just start walking on the moving walkways. At worst case scenario, walking from B15 to B95 may take 10-15 minutes, maybe.

Looking at flight stats for today, TPA-DEN arrived at Gate B47 10 minutes early, and DEN-MFR left from Gate B54 10 minutes late. I think even if the DEN-MFR flight weren't delayed 10 minutes, you would have had no problem making that connection today.

Of course, if you're one of those people who worry about tight connections, have you thought about an itinerary on AS? I see a TPA-SEA-MFR routing, though it's with a 3hr layover at SEA, and arrival into MFR after midnight.