CARACAS, Venezuela — Venezuela’s alliance of opposition parties declared on Wednesday that it would boycott the coming presidential election, saying the electoral system was rigged in favor of President Nicolás Maduro and his United Socialist Party of Venezuela.

The decision effectively cleared a path for Mr. Maduro’s re-election to another six-year term and threw the future of the nation’s political opposition, deeply weakened by recent election losses and internal fractures, further into doubt.

The coalition’s announcement emerged from weeks of debate following the decision in January by the all-powerful Constituent Assembly to put the election on an accelerated timeline, a move denounced by the opposition. The date was cemented this month by the country’s electoral commission.

The coalition had been divided between those who argued that participating in the April 22 election would end in certain failure at the polls and legitimize Mr. Maduro’s rule, and those who insisted that participation was an opportunity, however remote, for change.