BARCELONA—The emergence of Spain’s upstart populist party Podemos could hurt Catalonia’s independence movement by diverting attention from the industrially powerful region’s grievances with Madrid, Catalonia’s leader said.

As he prepared to embark on a trip to the U.S. to promote his independence campaign, Catalan regional President Artur Mas talked in an interview with The Wall Street Journal about the challenge of drumming up international support for his region’s uphill bid, ebbs and flows in foreign investment and anti-Catalan...