Changes... will be substantial," Sachin Pilot, corporate affairs minister and a star in Gandhi's team, told Reuters.
Critics say Gandhi depends on his family name for power, is a lightweight and has barely registered his presence in parliament despite being a member for the last decade.
"It is only in a personality and family-dominated set-up like the Congress that he can be nominated as the unquestioned supremo," said Arun Jaitley, a senior leader of the main opposition Bharatiya Janata Party (BJP).
The Nehru-Gandhi family has dominated politics in the world's biggest democracy ever since his great-grandfather Jawaharlal Nehru delivered his stirring "tryst with destiny" speech on the eve of independence from Britain in 1947.
The assassinations of prime ministers Indira Gandhi, Nehru's daughter, in 1984 and her son Rajiv in 1991 added an air of tragedy to the clan. Rahul is the son of Rajiv Gandhi.
Gandhi is a vice-president of Congress and was in charge of the party's campaign in state elections in 2013, in which it fared disastrously.
The Hindu nationalist BJP won three of five state assembly contests and its leader Modi remains the clear front-runner thanks to pledges of decisive leadership to revive economic growth that under Congress fell to its slowest pace in a decade.
(Read more: India's Congress party punished in state elections)
Earlier this month, Prime Minister Manmohan Singh conceded the government had failed to create manufacturing jobs, control inflation and combat corruption. He said he would step down and strongly suggested the reins be handed over to Gandhi.
Gandhi has in recent months tried to style himself as a maverick fighting corruption. But the new Aam Aadmi (Common Man's) Party (AAP), which formed the government in Delhi state after local elections last month, appears to have the lock on the anti-corruption and clean governance vote.
Indians from students to business executives are flocking to the party, inspired by its promise to clean up politics and the symbols of power that ordinary people have come to detest.
"Every establishment party is going to hurt," said Rural Development Minister Jairam Ramesh, a Congress strategist.