You can walk to T2 where Delta SC is located from either T1 (where most United flights arrive) or T3 without leaving secure area. The walk is 15 min at most; maybe 20 if you walk really slowly from the farther gate. Some United Express flights even operate out of T2, so there is a shuttle between gates C9 and E2.

If you arrive on an international flight, you will have to take a train to domestic terminals and go through security, but you can clear security at any domestic terminal (T1/2/3) regardless of where your flight actually leaves from.