"The current negotiations that are under way are definitely flawed," said Velayati, a conservative former foreign minister who is standing against Jalili in the June 14 election.
Velayati served as Iran's top diplomat for 16 years before being appointed an adviser to supreme leader Ayatollah Ali Khamenei, who has the final say on nuclear policy.
The nuclear programme has for years been a point of contention between the Islamic republic and the P5+1 countries of Britain, China, France, Russia and the United States, plus Germany amid suspicions that it is cover for a weapons drive.
Known for his tough stance since he took over as Iran's nuclear negotiator, Jalili has vowed to pursue the same tactic of "resistance" if elected president.
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Velayati said the approach had cost Iran unnecessarily damaging international sanctions.
"Diplomacy is not only roughness and hardiness. It is bargaining," he said.
"The art of diplomacy is that we save the nuclear right, while at the same time, we reduce the sanctions."
Jalili's negotiating style has also come in for criticism from another fellow candidate Hassan Rowhani, who headed the relatively moderate nuclear negotiating team that served under reformist president Mohammad Khatami in the early 2000s.