Northern cardinals are common throughout the eastern United States from Maine to Florida and continuing west to southern Minnesota, Nebraska, Texas, and the eastern half of Mexico showing the "northern" in their name is a bit of a misnomer. They are common in South Florida which is pretty much as far south as one can get.
Cardnials prefer dense woodland cover and riparian areas but can easily be found in suburban regions if mature trees are available. Northern cardinals are not considered threatened or endangered but are subject to a variety of hazards such as a attacks by outdoor or feral cats, overuse of insecticides or other chemicals, and window collisions as is true for most birds in general.
Because these birds are so adaptable and readily visit bird feeders, the northern extent of their range is actually expanding. As more birders offer supplemental food in the winter, these birds stay in less hospitable locations throughout the year.