DES MOINES — At the very first event of her first full day in Iowa as a presidential candidate, Senator Kamala Harris was greeted by a Democratic voter who delivered a pointed recommendation about the best path to the White House.

The voter, Rahul Parsa, who teaches at Iowa State University’s business school, said he told Ms. Harris at a gathering of Asian and Latino activists that she needs to think of the Democratic Party in the Midwest like a struggling retail business — and that she should seek out not just the regular customers, but those who are not loyal supporters as well .

“Kamala needs to find out why the people voted for Trump, what are their issues?” said Mr. Parsa, adding that he had one overarching demand for the burgeoning field of would-be nominees: “You need to bring some states in the Midwest.”

As the Democratic race takes flight, with one or more candidates entering the race almost every week, Mr. Parsa’s viewpoint represents one side of a long-simmering debate within t he party : Should Democrats redouble their efforts to win back the industrial heartland that effectively delivered the presidency to Donald J. Trump? Or should they turn their attention to more demographically promising Sun Belt states like Georgia and Arizona ?