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ASUNCION (Reuters) - Paraguayan Senator Mario Abdo Benitez, a lawmaker with ties to a former Paraguayan dictator, won the ruling Colorado Party’s presidential primary on Sunday in a sharp rebuke for President Horacio Cartes, the party’s flagbearer.

Abdo, the son of former dictator Alfredo Stroessner’s private secretary, beat out ex-finance minister Santiago Pena, a young economist and Cartes’ hand-picked successor, in a surprise outcome that followed increasing dissatisfaction with Cartes, who has been in power since 2013.

Abdo will face lawyer Efrain Alegre, who won the opposition Liberal Party’s nomination, in the April presidential election in the land-locked nation of 6.8 million, long one of South America’s poorest.

The Colorado party has governed Paraguay for nearly 70 years, including during Stroessner’s 1954-1989 dictatorship, with a brief interruption when leftist Fernando Lugo was elected in 2008 and impeached in 2012.

With over 96 percent of primary votes counted, Paraguay’s electoral authority said that Abdo had received 50.9 percent of the vote. Pena had garnered 43.3 percent.

“The arrogant establishment has been defeated today and forever ... we all have scars but are urging unity for the Colorados and later, Paraguay,” Abdo told his supporters following his victory.

Cartes, a former soft-drink and tobacco executive, has run a low-tax policy that investors credit with spurring one of the fastest economic growth rates in Latin America. But his attempt to change laws enabling him to run for re-election spurred protests and was widely criticized.

Abdo, known in Paraguay by his nickname “Marito,” has been a harsh critic of Cartes, and aligned with the opposition in Congress to fight his proposed re-election amendment.

More than one million members of the Colorado Party voted on Sunday, an estimated turnout of 50 percent.