BUDAPEST — 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.

The private meetings, recounted by two people who were in the room and by a third who was briefed on the discussions at the time, occurred in early May 2010. Nearly eight years later, Mr. Orban has remade Hungary’s political system into what one critic calls “a new thing under the sun.” Once praised by watchdog groups as a leading democracy of post-Soviet Eastern Europe, Hungary is now considered a democracy in sharp, worrisome decline.

Through legislative fiat and force of will, Mr. Orban has transformed the country into a political greenhouse for an odd kind of soft autocracy, combining crony capitalism and far-right rhetoric with a single-party political culture. He has done this even as Hungary remains a member of the European Union and receives billions of dollars in funding from the bloc. European Union officials did little as Mr. Orban transformed Hungary into what he calls an “illiberal democracy.”