CARACAS—Venezuelan President Hugo Chávez soared to a bigger-than-expected victory over his challenger in elections Sunday, surviving the biggest test of his 14 years in power and handing him another six-year mandate to rule the oil-rich nation.

With 90% of the vote counted, official results showed Mr. Chávez and his United Socialist Party of Venezuela winning 54.4% of the votes compared with 45% for Henrique Capriles, a former state governor who ran a spirited campaign for change. Turnout was a surprising 81%, the election...