BUENOS AIRES—The death of Argentina's ex-president Néstor Kirchner, seen by many as the power behind his wife's government, creates vast political uncertainties but also opens the possibility that the country could shift away from its combative populist policies.

Mr. Kirchner, 60, died early Wednesday of a heart attack while visiting the southern city of El Calafate with his wife, President Cristina Kirchner. The fiery former governor served as president between 2003 and 2007, and was widely expected to seek another term in...