JAIPUR, India — As the once-formidable Indian National Congress party began to crumble in recent years, Sachin Pilot, a young, well-connected politician, stepped up to help his bloc weather one crisis after another.

Mr. Pilot’s parents were Congress stalwarts and served in elite government positions. He is close with members of the Gandhi family, who have controlled the party for decades, dominating India’s electorate until Prime Minister Narendra Modi first swept into office six years ago on a wave of Hindu nationalism.

Many thought Mr. Pilot, 42, who represented Congress as deputy chief minister in the northern state of Rajasthan, had the perfect résumé to lead the party to a rebirth.

Instead, India’s political establishment has been jolted by accusations this month that Mr. Pilot was secretly working on behalf of Mr. Modi’s Bharatiya Janata Party, possibly in an effort to depose a chief minister he had fallen out with. Mr. Pilot has denied those accusations.

Congress loyalists fear the scandal is another severe blow to the survival of the party, which led the country to independence more than 70 years ago but has been cracking internally under the weight of huge Bharatiya Janata Party victories led by Mr. Modi. Party officials in Rajasthan immediately called for dozens of Congress politicians to move into a hotel in a desperate effort to keep them from defecting.

As the country’s political discourse has been dominated by nationalism and Hindu-centric policies, many also worry that the weakness of Congress, which traditionally has championed minorities, harms India’s democracy more broadly.

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The scandal broke out in recent days when party officials accused Mr. Pilot of offering multimillion-dollar bribes to fellow Congress leaders and urging them to defect to the Bharatiya Janata Party over a period of several months.

The fallout was swift and unforgiving. Party leadership removed Mr. Pilot from his post and accused 18 other politicians of colluding with him. Ashok Gehlot, the chief minister in Rajasthan, called his deputy a “worthless person” and released audiotapes that he said implicated Mr. Pilot in a conspiracy to buy off officials “like goats in a market.”

“Speaking good English, giving good quotes and being handsome isn’t everything,” Mr. Gehlot told reporters.

Mr. Pilot, who did not respond to requests from The New York Times for comment, maintained his innocence in an interview with India Today, insisting that he had no intention of joining the Bharatiya Janata Party. He accused Mr. Gehlot of trying to “humiliate me” and block him from a bigger role in the Congress party.

It was one of the most public expressions yet of resentment within Congress that the party is failing to make room for a younger group of leaders to emerge. The upshot is that Mr. Modi’s party is stronger than ever.

Gilles Verniers, an assistant professor of political science at Ashoka University, said Congress has failed to give incentives to officials to stick with the party, leading some to “seek greener pastures.” In recent years, dozens of senior Congress leaders have left the party, fed up with a lack of visionary leadership.

Mr. Modi’s B.J.P. has seized on the turmoil, steadily undermining Congress in the few states that it still controls. In Madhya Pradesh, Jyotiraditya Scindia, another Congress star who was close to the Gandhis and came from a long line of princes, defected to the B.J.P. in March, causing Congress to lose its fragile hold on the state government.

In an interview, Satish Poonia, the president of the B.J.P. in Rajasthan, said “all of the allegations against us are false.” Instead, he described the conflict as a bitter generational rift between Mr. Pilot and older Congress leaders like Mr. Gehlot, 69 — a view also shared by many within Congress.

India has a long history of power grabs and high-level defections, and local news outlets have published accounts from unnamed B.J.P. officials in Rajasthan describing a political coup attempt gone awry, with a disgruntled Mr. Pilot at its center.

Rajasthan is a large, politically competitive state, and home to some of India’s vital heritage and cultural sites. Congress controls 107 seats in the state legislature, barely enough to form a majority government. Just a few crossovers would shift the balance of power to the B.J.P.

Congress leaders in Rajasthan are deeply rattled. Last week, Mr. Gehlot’s government moved more than 100 elected officials into the Fairmont Hotel, a five-star resort in the city of Jaipur, the capital of Rajasthan, hoping to prevent them from leaving for the B.J.P. The fear was that if the lawmakers remained at home, it would be easier to bribe or entice them to defect.

On a recent day this week, riot buses and police officers dressed in camouflage blocked the resort’s entrance. Inside, politicians attended yoga classes and watched Bollywood movies in a marble-floored lobby.

Rafeek Khan, a member of Rajasthan’s Legislative Assembly who is among those staying at the hotel, said he had no idea when they would be allowed to leave.

“The B.J.P. is playing with democracy,” he said. “They think one can be bought if enough money is offered. That is the message the B.J.P. is sending to the electorate.”

In the coming days, Rajasthan’s legislature is expected to decide whether Mr. Pilot and the 18 other rebel officials will lose their seats, which would make forming a new government under the B.J.P. much tougher.

The decline of Congress accelerated last year, when Rahul Gandhi, the great-grandson of India’s first prime minister, Jawaharlal Nehru, resigned from his post as president after a humiliating general election defeat against Mr. Modi.

Many Indians see the Congress party as a protector of the nation’s founding secular values, but far too beholden to the Gandhis, who are often perceived as a symbol of India’s Anglicized upper class. Mr. Gandhi, his mother, Sonia, who assumed the role of interim president, and his sister, Priyanka, all still call the party’s shots.

Mr. Modi continually hammers Congress for being a family dynasty, and his party’s embrace of a brawny Hindu nationalism that emphasizes India’s Hindu heritage has tracked well with voters.

Rasheed Kidwai, a former journalist who wrote a book about Sonia Gandhi, said Congress simply could not compete. “They are going through the motions,” he said. “They are very demoralized. They don’t have any hope.”

Analysts say trouble has been brewing for Congress in Rajasthan since 2018, when the party scored a victory over the B.J.P. in state elections and retook control of the state government after five years in the opposition.

In his interview with India Today, Mr. Pilot said he proposed himself as chief minister, citing his role in helping secure that win. But he ultimately relented to an agreement to take the second spot behind Mr. Gehlot under the impression that he would soon be elevated.

Mr. Pilot, whose name comes from the fact that his father was a decorated combat pilot, apparently felt sidelined by Mr. Gehlot, and bitterness between the men grew.

“This was a badly handled show and one more state could slip out of Congress’ hands,” said Neerja Chowdhury, a political analyst in New Delhi. “It is a failure of national leadership.”

Hari Kumar and Jeffrey Gettleman contributed reporting from New Delhi.