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Phidippus cardinalis
cardinalis presumably refers to cardinal red, an allusion to the dorsal color. (information from BugGuide)
Mixed prairie in a City park
I had been comparing those two species after getting these shots, and I was leaning toward P. cardinalis as well. Thanks to both of you for the ID assistance and to Marta for the kind words. :-)
The reason I believe it to be cardinalis over apacheanus are the spots on the abdomen and the female apacheanus tend to have more distinct bands on the legs.