Phil Allison wrote in
There is no guarantee that a relay specified for use in an automotive,
12V DC environment is suitably insulated for connection to the AC
supply - where spike voltages in the thousands occur.

An excellent point however in this instance the controlling circuit was to
be another 120V circuit with the only reason for 12V being involved at all
was the availability of the relay. I would have just used some old 12V
charger from the junk box. So if hypothetically some big spike had come
down the switched line, jumped over to the coil circuit, and blown through
the P/S it would just have ended up on another 120V circuit which is just
as prepared or not prepared for that as any other. Likewise spikes coming
the other way.

However, it's a moot issue anyway as I realized I need NC contacts while
the 12V relay is SPST NO. Most likely I will dig up a DPDT or 4PDT and
parallel the poles for added ampacity. My "wtf was I thinking" is all
about forgetting I need NC not about why I thought the 12V relay might be
suitable.