India's Congress party has rallied round south Asia's most famous political dynasty after exit polls showed it was heading for a historic defeat in the general election. Despite the public display of support, the apparently disastrous result has led some to predict the end of the Nehru-Gandhi family's decades-long dominance of Indian politics.

"The dynasty per se is finished. It will be a long slow death but it is incredibly difficult to see them recover from here," said Ramachandra Guha, an Indian political historian and commentator.

Though such surveys have proved wrong before, polls released by major TV channels on Monday – the final day of the five-week election – tally with other surveys during the campaign and the contending parties' internal reporting.

The polls suggest a huge win for the opposition Bharatiya Janata party, led by the controversial Hindu nationalist Narendra Modi, with centre-left Congress reduced to what could be its lowest total since India won its independence from Britain in 1947. Final results will be announced by the election authorities on Friday.

The Congress campaign was led by Rahul Gandhi, the 43-year-old vice-president of the party and son, grandson and great grandson of prime ministers.

Gandhi's mother, Sonia, is president of the Congress party and widely considered the most powerful woman in India. His father, Rajiv Gandhi, and grandmother, Indira Gandhi, were both prime ministers and were both assassinated. His great grandfather Ja waharlal Nehru is one of the founding fathers of the modern Indian nation.

"They have been out of power before but this time the failure of the leadership is very obvious. Their prestige will certainly be seriously weakened," said Mohan Guruswamy, of the Centre for Policy Alternatives, a Delhi-based think tank.

Gandhi, a former management consultant who has no experience of government, is but seen as a poor public speaker and lacking in charisma.

"Rahul Gandhi has been rejected for perhaps the nth time by the electorate. In his decade-long career, it is difficult to recall even a single instance where Rahul has delivered for the Congress," wrote Sandipan Sharma in the Hindustan Times newspaper.

The BJP's own internal polling has suggested that Gandhi may even lose his own seat of Amethi, a family bastion for decades. Such a result would destroy his credibility within the party.

Senior officials of Congress, which has held power in India for 49 of the last 67 years, have already moved to protect the senior leadership from blame for the defeat, arguing that any failure was "collective".

"Whatever be the result, it will be a collective responsibility of all the leaders," Shakeel Ahmed, a general secretary of Congress, told reporters.

Ahmed said exit polls had been wrong in 2004 and 2009 and "are positively disposed towards BJP".

Kamal Nath, a Congress minister, said election results were "a reflection on what people's perception of the government functioning was ... Rahul Gandhi was never part of the government."

A second minister who did not want to be named told the Guardian the campaign had shown the name of the Gandhi family was still "very much attractive" to voters.

"The common people understand how much [the Gandhis] have sacrificed for their nation. They have been assassinated, killed themselves working for India, with only the best interests of all citizens and inhabitants in their hearts," he said.

Though Rahul Gandhi himself has worked for more democracy within the Congress Party itself, it seems unlikely that he or other members of his family could be ousted from their present leadership positions, suggesting the possibility of a long period in opposition.

"Habits of deference and sycophancy dependence die hard. It is possible the Congress could revive under a different leadership but very difficult to see who would lead any revolt," said Guha, the historian.

Before the campaign began Congress officials in Lucknow, the state capital of Uttar Pradesh, said a Congress party not led by the Gandhi family was "inconceivable".

Many point to Priyanka Gandhi, Rahul's younger sister, as a potential hope for the party. The 42-year-old is seen as an effective campaigner but has avoided, or been prevented from taking, top leadership roles.

Ashok Malik, a Delhi-based analyst, said if the Gandhi dynasty had taken Congress to a historic low, it raised significant questions about its long-term future.

"What's the next card they can play? If Amethi is lost then Rahul is finished. Priyanka is a better speaker but is she a better politician?," Malik said.

Congress has failed to adjust to huge changes in India in recent decades, where younger voters are now more impressed by the opportunities a candidate appears to offer them than the achievements of his forebears, Malik said.

During the campaign Modi, at least at a national level, played down his party's traditional commitment to religious and cultural revivalism in favour of stressing development, jobs and honest government.

The 63-year-old is however polarising figure who has been accused of failing to stop, or even encouraging, sectarian violence in 2002 in Gujarat, the state he has run for 11 years. About a thousand people, largely Muslim, died in rioting after 59 were killed in an arson attack on a train carrying Hindu pilgrims. Modi denies any wrongdoing and judicial investigations have found insufficient evidence to support the charges against him.

Few of India's 150 million Muslims are expected to have voted for the BJP.

Gandhi repeatedly called the election a battle of ideas and has contrasting his "inclusive, empowering" vision of India with the divided nation that he says Modi's victory would mean.

But this approach does not appear to have resonated with voters after years of flagging growth, price rises and widely reported graft scandals since Congress won power in 2004.

Milan Vaishnav, an expert in Indian politics at the US-based Carnegie Endowment for International Peace, said both rural and urban areas had seen swings to the BJP that were key to its apparent success.

Congress has traditionally done well in the countryside and the outgoing government had focused a series of major subsidy programmes on poor rural voters.

"If the poll results are confirmed you are looking at a major realignment. The absence of a leader able to project a vision hurt [Congress]," he said.

Party grandees are also seen as out of touch. One mocked Modi – who once helped his father sell tea from a roadside stand before rising to be chief minister of the major state of Gujarat – for his humble origins.

Ravi Shankar Prasad, deputy leader of the BJP, said on Monday Indians had rejected "arrogance, dynasty and inheritance".

"Dynasty cannot deliver victory. The Congress should understand … you can't flaunt your inheritance and achievements of your forefathers attained 20 to 30 years ago," Prasad said.

The election saw record participation levels, shattering the previous record of 417 million set in India five years ago, according to the election commission. In some areas in earlier rounds about four-fifths of those eligible cast a ballot. Overall almost exactly two-thirds of eligible voters turned out.

The exit polls put the BJP on 270 to 280 seats with more than 35% of the vote, and Congress with about 20% of the vote and 110 seats. The BJP and its current allies need 272 for a majority in the powerful lower house.

Indira Gandhi, Nehru's daughter, is widely blamed for instituting the dynastic tradition in Congress. Nehru's only child, she took her family name from her husband, a minor politician, and had no blood relation with the famous Mahatma. A four-time prime minister, she promoted her two sons. One died in an accident but the other, Rajiv, rode power on the wave of sympathy that followed her death in 1984.

Congress was out of power for six years before winning elections in 2004 Sonia Gandhi, Rajiv's Italian-born widow, is widely credited with reinvigorating the organisation and winning the victory. She is now 67 years old and has undergone medical treatment in the US for an undisclosed ailment.

Earlier this year, Rahul Gandhi told an interviewer that he would take a phlegmatic view of defeat.

"I don't go into an election thinking, if we lose it's the end of the world. We lose some elections, we win some elections. The real thing is that it's a heart thing. It's a soul thing," the Cambridge-educated politician said.