"I have great respect for him (Aiyar) but I don't agree with his statement that Congress should sit in Opposition. It is defeatist mentality... It demoralises party cadres," said the veteran Congress leader.
Asserting that Congress was contesting the upcoming Lok Sabha elections to win and not lose, he said, "I did not expect him or anybody else to say such a thing."
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He was commenting on Aiyar's remarks made some time ago when he had said that Congress should "sit in opposition and re-invent itself".
Singh, who was a Union minister before becoming the Himachal Chief Minister in 2012, insisted that the UPA government led by Manmohan Singh has done a "wonderful work" and there was nothing to be defensive about.
The 80-year-old leader went on to add that Congress governments earlier also have done great work and "what we see now is the result of persistent efforts by Nehru, Shastri, Indira Gandhi, Narasimha Rao and Manmohan Singh governments".
He acknowledged that Narendra Modi had got a headstart in the campaigning for the Lok Sabha polls but said Congress can make up.
"Although BJP started much early, it is not necessary that we cannot catch up. If we have an effective campaign, we can make an impact... But it is immaterial as to who started first, the important part is who finishes first," he remarked.
He said Congress has so far been campaigning in its traditional style.
"While we are using traditional ways of campaigning, Modi is using western style, the presidential style of campaigning. Congress should bring out facts and match the BJP campaign, particularly in style," Singh said.