“It is just a lie,” he said. “It is a campaign trick.”

Mr. Ponta led a field of 14 candidates in the first round of voting on Nov. 2 with 40 percent, drawing most of his support from rural areas in eastern and southern Romania, but also winning Bucharest, the capital. Mr. Johannis, whose support was largely in the more prosperous Transylvanian region in the west, was second with 30 percent.

For a decade, one of the central political battles in Romania has been over the establishment of independent prosecutors capable of uprooting corruption, said Sorin Ionita, chairman of the Expert Forum, a public policy research group in Bucharest. More than 1,000 public officials, including former Prime Minister Adrian Nastase, one of Mr. Ponta’s mentors, have been convicted of corruption in recent years.

Mr. Ponta vows to continue the anticorruption campaign, which he credits with helping Romania overcome its budget deficit by cracking down on tax cheats. But analysts worry that, despite his promises, he will slowly chip away at the system.

“They don’t need to do it right away,” Mr. Ionita said. The first order of business will be to reassure Brussels and Washington, he said, and hope that international pressure eases.

“Then, appointing chief prosecutors who are less independent, a few at a time, you can slowly erode the effectiveness of the institutions,” Mr. Ionita said.

Mr. Ponta’s political orientation is neither East nor West, said Adrian Moraru, deputy director at the Institute for Public Policy in Bucharest, and recent overtures the prime minister has made toward China may be calculated simply to keep his options open with Brussels.

“He is a pragmatist,” Mr. Moraru said. “If he ever has his back to the wall and needs to turn to China or Russia, I don’t think he will have a second thought.”