Trump weakening? Not judging by his Iowa swing.

Braving the cold March Iowa wind, Patty Havill stands amid a snaking line of red MAGA hats and flag-themed costumes, waiting to see the one man she believes can still save America from all its problems. Other Republican presidential hopefuls have been through town recently – including, on Friday, hot-ticket Florida Gov. Ron DeSantis – but Ms. Havill is single-minded in her devotion. For her, it’s Donald Trump or no one.

“Our country is in chaos. We need him to straighten it out,” says the retired nurse. “Trump is the only one.”

Since...