I flew SFO-ORD-MAN with AA a few weeks back. We had a similarly timed connection in ORD and were fully prepared for a reroute through LHR, but we arrived on time, disembarked, crossed the corridor to the opposite gate and actualy had to wait the whole half hour in the lounge instead.
A reroute through LHR is probably only going to cost you the same amount of time as it will to wait for the next PDX flight anyway, so worth going for IMHO.