Congress, which has ruled India for more than 50 years with centre-left policies, has struggled in the past decade to compete with Modi's Bharatiya Janata Party (BJP) that counts the country's Hindu majority as its main vote base.

The seven-phase election starts on April 19, with vote counting on June 4.

More help for so-called backward castes and the poor irrespective of caste were among Congress pledges in its election manifesto, including assured jobs for the young, guaranteed prices for farm produce and higher health insurance.

"Congress has been the most vocal and active champion of the progress of the backward and oppressed classes and castes over the last seven decades," said the manifesto released by party president Mallikarjun Kharge, who himself is from one of India's most backward castes.

"However, caste discrimination is still a reality."

Kharge listed out some of the key pledges with only a feeble response from party workers at a press conference, leading him to comment: "No claps, nothing!"

Many analysts say morale is low in Congress because of the BJP's dominance of the country's politics. An opinion poll released on Wednesday predicted Modi's National Democratic Alliance coalition could win 399 of the 543 seats in the lower house of parliament while the BJP alone is projected to win 342.

Congress could fall to 38 seats, a record low, it said.

Former Congress chief and frontline leader Rahul Gandhi said the election is "much closer than being propagated."

"It is a close election and we are going to fight an excellent election and we are going to win the election," he said at the event.

Congress said should it be voted back to power, it would conduct a nation-wide socio-economic and caste census to "strengthen the agenda for affirmative action", guaranteeing a constitutional amendment to raise the 50% cap on reservations for backward castes in government jobs and education.

The party said such groups make up nearly 70% of India’s 1.42 billion people but "their representation in high-ranking professions, services and businesses is disproportionately low".

The caste system has set out hierarchies in the Hindu religion for thousands of years, but it has been countered by affirmative action policies in recent decades albeit with uneven effect.

Average monthly spending by marginalised castes from India's so-called scheduled caste, scheduled tribe and other backward classes lagged privileged castes by 27% in rural and 30% in urban India in 2022/23, according to a government survey released last month.

Such a spending gap has narrowed in over a decade but much more slowly in India's distressed countryside where a majority of the nation's population lives, according to a comparison with data from a similar 2011/12 survey.

Data on monthly spending is a key component in India's poverty calculations and helps assess economic status.

India's poorest and populous state, Bihar, has held a caste-based census which underlines the skewed representation of marginalised castes in government jobs.

Its minority so-called upper castes hold nearly a third of valued government jobs, while 95% of the state's people are either self-employed or unemployed, according to the survey released last year.

Moreover, 88% of all poor families in Bihar, described as those earning up to $72 a month, are from marginalised castes, according to the survey seen by Reuters.

The BJP has dismissed such surveys. Modi has said in rallies this year that India's downtrodden castes are the poor, youth, women and farmers, and that he was working tirelessly for them.

(This story has been refiled to remove an extraneous word in paragraph 1)