Prime Minister Viktor Orban has remade the country’s political system and pioneered a model of one-party rule. His actions can be seen as part of a broader decline of democracy. UDAPEST — The senior leaders of Fidesz gathered on the banks of the Danube, in a building known as the Hungarian White House, stunned by the scale of their good fortune. Their right-wing party had won unexpectedly sweeping political power in national elections. The question was how to use it. Several men urged caution. But Viktor Orban, the prime minister-elect, disagreed. The voting result, Mr. Orban continued, had given him the right to carry out a radical overhaul of the country’s Constitution. Mr. Orban won the argument.