Is that international->international, or domestic->international?
If the latter, it may well be possible (although still unlikely). Check UA's own website and see if the connection comes up - if it does it's legal and should be OK, although your bags will probably be at greater risk of misconnecting than with a longer transfer time. Domestic->international will not require you to collect and re-check your bags.
Despite my own distaste for UA, they did pull off one of the more impressive baggage handling feats at SFO and LAX once when I was re-routed on the way back to LHR - despite being booked onto an SFO-LAX that was already late and ready to go (and which went inside 20 minutes) and then onlining onto an LAX-LHR with a 35 minute connection time, my bag did follow me onto both flights.
If it's an international->international connection, I don't see how it can be done. You need to clear immigration and customs and then re-check - no direct airside transfers in the US any more.