ST. CHARLES, Mo. — The prosperity is apparent on the way into town: the 21-floor casino resort and spa on one side of the interstate, and on the other a freshly built retail quarter of boutiques, a brand-new Hilton hotel and a P.F. Chang’s. It unfolds from there along the highways heading west with more gleaming office parks and multiplying subdivisions.

This is not the Trump country of the popular imagination, the land of shuttered plants and the economically left behind. St. Charles County, in the suburbs northwest of St. Louis, has had the highest median household income in Missouri for several years.

But in 2016, Donald J. Trump won the county by 26 points, and he is still popular among people like Tom Hughes, a homebuilder whose business was rebounding from the recession before Mr. Trump took office.

But, Mr. Hughes said, “Now there’s an optimism that I haven’t seen, maybe ever.”

Mr. Trump rode to office in part by promising economic revival to sputtering towns across America. Economic growth has accelerated since he took office, from the final year of President Barack Obama’s administration, and Mr. Trump frequently claims credit for it.