Q.Belly-up June Beetles
Belly-up June Beetles began appearing on my back porch a couple of days ago. The porch is wooden, about 3 to 4 feet off the ground. My wife brought home a hanging flower pot a few days ago. Is that the source of the beetles? I’ve never seen them before now. They show up in the morning, apparently dead or dying; some of them are moving their legs a bit. By evening they are gone. I speculate cardinals are picking them up.

It's possible but could be merely coincidental. What plant was it that your wife brought home? For more information on june beetle control, please visit the following link:
https://www.gardeningknowhow.com/plant-problems/pests/insects/june-bug-beetles.htm