BY JEREMY PAGE AND MARK PETERS

MUSCATINE, Iowa—This small city on the Mississippi River has long boasted that Mark Twain briefly called it home in 1854. Now, residents realize they have a more unusual bragging point: Muscatine played a minor but memorable role in the ascent of Xi Jinping, the man expected to become China's top leader this fall.

Twenty-seven years ago, Mr. Xi, then an up-and-coming official in a pig-farming region in China, led an animal-feed delegation to Iowa. He toured farms, visited a Rotary Club and watched a baseball game. He spent two nights in the split-level home of a Muscatine couple, sleeping amid ...