Thanks very much for any insight you can provide. I'll answer all your followup questions below:

Voltage across the posts is currently 12.3V. When this first occurred earlier in the day I believe the voltage read around 12.4V. Turning the key to ACC position does not change the voltage at all.

I have no power to windows, power locks, aux outlets, or clock in the ACC key position or any other key position. When turned to start position, the starter does not turn either.

The previous owner had a keyless entry unit installed (model: Directed 4x03) which I never used or made adjustments to. It is currently blinking a red indicator light. There is no evidence of a dedicated immobilizer from my searching, but I do have the Toyota security light on the panel near the driver's side fuse panel. It is currently unlit.

I just tested voltage between ground and the ACC pin of the ignition switch wiring harness. Voltage rises from about 3.4V at the ACC pin to 12.2V when key is turned to ACC position. Would this suggest that the contacts are fine?