ASUNCIÓN, Paraguay — Horacio Cartes, a Paraguayan tobacco magnate, faced various challenges during his presidential bid. He was pressed to explain why antinarcotics police officers apprehended a plane carrying cocaine and marijuana on his ranch in 2000; why he went to prison in 1989 on currency fraud charges; and why he had never even voted in past general elections. till, voters across the country seemed ready to give Mr. Cartes the benefit of the doubt, handing him a solid victory in Paraguay’s presidential election on Sunday. He took 46 percent of the vote against 37 percent for his main opponent, Efraín Alegre of the ruling Liberal Party, with about 80 percent of the voting stations reporting. Electoral authorities declared Mr. Cartes the winner. Mr. Cartes’s victory returns the presidency to the conservative Colorado Party — which held a tight grip on power for six decades, until 2008 — and opens a new phase of international scrutiny of Paraguay, the landlocked nation with a long reputation as a haven for smugglers.