Mani Shankar Aiyar - a veteran of the party who is close to the Nehru-Gandhi family that has run the Congress for decades - predicted defeat in the national election due by next May.
"Who can be even half-way realistic and expect the Congress to return to power" Mani Shankar Aiyar said.
In a further signal of uneasiness about Congress' chances in the general election, the leader of a key party in the coalition government said on Monday the state polls amounted to a rejection of "weak rulers".
"People do not want weak rulers," National Congress Party leader Sharad Pawar said, according to domestic media reports. "They want decisive and result-oriented leaders who will formulate policies for (the) poor and implement them."
Manmohan Singh, the taciturn 81-year-old prime minister, has been widely criticised for the government's policy drift and a sharp economic slowdown, and for allowing corruption to spin out of control since he was appointed to a second term in 2009.
Adding to the Congress party's troubles, half a dozen of its own lawmakers called for a parliamentary motion of no-confidence over a decision to split the southern state of Andhra Pradesh into two.
If at least 50 members of the lower house back their demand, the stage would be set for a trial of strength in which Congress would need the support of several parties to survive.
Mani Shankar Aiyar's remarks follow a disastrous showing for the Congress party in elections held over the last month in three big states and the capital, Delhi.
"A break from governance would be a welcome break that could be used to refit the party as the nation's natural party of governance in the 21st century," the upper house lawmaker wrote in a column for the Indian Express newspaper.
"The current and prospective electoral reverses for the Congress are thus Rahul's golden opportunity," he said, referring to the young scion of the Nehru-Gandhi dynasty and potential candidate for prime minister.
Mani Shankar Aiyar's opinion was personal and not the party's view, said Congress spokesman Bhakta Charan Das, but agreed there was a need to analyse what went wrong.
"The party will definitely introspect and we must come out with a very good approach to revitalise ourselves."
The centre-left Congress party's main opponent, the Hindu nationalist Bharatiya Janata Party (BJP), was the clear winner in three states that held assembly elections and left its rival standing in the capital.
The BJP has been boosted by the energetic campaigning of its charismatic candidate for prime minister, Narendra Modi, but also by voter fatigue with Congress after years of spectacular corruption scandals and stubborn inflation.
Aiyar said the growing strength of regional parties would make it hard for the BJP to form a stable coalition and predicted there would be new general elections by 2016.
"I am deeply convinced that, whoever forms the government in 2014, we will be faced with another general election by 2015 or 2016, at the latest," he said.