Singh said the prime minister worked for the poor, farmers and weaker sections of society during the past four and a half years of his tenure.
Addressing an event in Moradabad, he cited several central government schemes — such as Ujjwala Yojana of providing free LPG connections to poor households and Ayushman Bharat health care programme — to make his point.
"Chowkidar chor nahi, chowkidar pure hai. Next PM sure hai, problem ke liye cure hai," Singh said, using the word Congress president Rahul Gandhi has used to attack Modi on the Rafale jet deal.
Singh's statement in English meant: "The watchman is not a thief, he is a pure man." "He is sure to be the next PM, he is a cure for problems."
The Congress has been alleging that the 2015 Rafale deal to purchase 36 fighter jets from France was compromised by corruption.
Gandhi has repeatedly alleged Modi helped industrialists in getting defence contracts related to the deal.
On Monday, the home minister responded to the allegations, saying the defence deal strengthened the Indian Air Force, which had been demanding fighter jets for 30 years.
The prime minister finalised the Rafale deal to strengthen India's security, he added.
Citing former prime minister Atal Bihari Vajpayee, Singh said he tested nuclear bombs in 1998 without caring about the warnings and threats from world powers and it was this courage that made India a nuclear weapons State.
He also recalled India's victory during the Kargil war with Pakistan.