In India’s last general election, in 2019, Prime Minister Narendra Modi’s party won 303 of 543 parliamentary seats — nearly six times as many as the main opposition party, the Indian National Congress. It was a stinging electoral blow for the Congress, a once-dominant party that has appeared greatly diminished in recent years, and exit polls in this year’s election had not suggested it would fare much better.

But early election results on Tuesday indicated a far stronger showing than expected for the Congress. The party and its allies were leading in nearly 230 races, a sharp turnaround that prompted jubilation at the Congress headquarters in New Delhi, where supporters erupted in cheers each time a television channel announced a new lead for one of its candidates.

“Whatever the final results, one thing is clear — it is a moral victory for Congress and our leader Rahul Gandhi, and defeat for B.J.P.,” said Robin Michael, a political worker, referring to Mr. Modi’s Bharatiya Janata Party.

While there was no indication that Congress and the opposition coalition it leads would scrape together a majority to unseat Mr. Modi, party workers said that they had dented Mr. Modi’s aura of invincibility. They praised Mr. Gandhi, the Congress party’s most prominent figure and a great-grandson of Jawaharlal Nehru, India’s first post-independence prime minister.

Last year, as Mr. Gandhi sought to burnish his standing by leading long marches across India, the B.J.P. ensnared him in a court case that led to his expulsion from Parliament. He was later returned to his seat by India’s highest court. On Tuesday, Mr. Gandhi was on track to win his parliamentary seat in the southern state of Kerala.

The Congress, long positioned at India’s political center, has struggled to find a direction and offer an ideological alternative to the Hindu nationalist B.J.P. It has faced rebellions, infighting and periodic fits of soul-searching over whether to rally behind a new face — only to stick with its dynastic leadership.

This year, despite expectations, Mr. Gandhi had set a target of doubling the party’s 2019 tally of 52 seats. By late Tuesday afternoon, it was leading in nearly 100 seats.

“We will stop Modi from making a mockery of this country and turning people against each other,” said Sandeep Mishra, a Congress worker at party headquarters. He added: “Indians are fed up with Modi.”