If the Hindu zealots break through the police cordon around New Delhi, or if riots blaze between Hindus and Muslims, then Mr Rao may lose the battle and be forced to resign. The 71-year-old Prime Minister faces a vicious revolt within his own Congress party, which rules India with a frail minority government. And, if Mr Rao's security forces, all 70,000 of them in Delhi, fail to halt the protest rally, it will be seen as a victory for the forces of Hindu revivalism now sweeping across northern India.
An outbreak of violence in Delhi, government officials warn, could spread like a brushfire across the country. When Hindu zealots tore down a mosque in Ayodhya last December, it scarred relations between the majority Hindus and the 120 million Muslims, and led to rioting that left more than 1,900 people dead, according to the government.
The rally is being led by the right-wing Hindu Bharatiya Janata Party (BJP), the main parliamentary opposition group, and it is backed by a number of Hindu extremist organisations and trident-wielding holy men. The BJP is demanding Mr Rao's resignation and fresh elections.
Mr Rao is not taking any chances. A nationwide crackdown of BJP party workers is under way. The BJP claims that in the past few days police arrested nearly 100,000 of its party workers, including 5,000 of its top cadres in Delhi. All trains into Delhi are being stopped and searched by police, and security forces have sealed all roads leading into the capital. Three stadiums have been turned into temporary jails. The army has been put on alert.
Originally, the BJP wanted 2 million Hindu militants to swarm across Raj Path, the wide boulevard leading from India Gate up to the government buildings and parliament. But now the heart of the capital is ringed with barbed-wire, and police tent encampments have sprung up on the parliament lawn.
Despite the police cordon, the BJP claims it can raise more than 150,000 Hindu militants to converge on Raj Path from different areas of Delhi. Several of the routes chosen by the BJP cut through Muslim neighbourhoods, and newspapers report that many Muslims living in predominantly Hindu areas are fleeing to the safety of the medieval walled city.
For two days running, right-wing Hindu MPs, angered by the rally ban and the arrest of so many party workers, disrupted a key parliamentary session on the budget. Pramod Mahajan, a BJP vice-president, said the government was 'panicking', and that the banned rally would be peaceful. Lal Krishna Advani, the party leader, said: 'We've told our workers that even if police with clubs attack them, they must not reply to violence with violence.'
The quarrel between the Hindu revivalists and the Congress government erupted after Mr Rao ordered the arrest of senior BJP politicians after the destruction of the Ayodhya mosque. Then, Mr Rao imposed direct rule from New Delhi on all four states with BJP-elected governments. The Prime Minister also banned several extremist Hindu groups, though in many states, Hindu police officers have been lax in enforcing the ban.
The Congress party, instead of closing ranks behind Mr Rao, is riven by dissent. The powerful chief minister of Maharashtra, Sudhakar Naik, was sacked because of his mishandling of the Bombay riots and because the Defence Minister, Sharad Pawar, was gunning for him. More worrying for Mr Rao is that several party bosses, led by the Human Resources Minister, Arjun Singh, have openly criticised the Prime Minister for being too lenient on the Hindu extremists.
Until now, Mr Rao has been able to rally the left-wing parties, which prop up the minority government, under the banner of secularism and democracy to do battle against the BJP's religious politics. But these parties are losing confidence in Mr Rao for having let India slide this far into turmoil.