Personal law in India — marriage, divorce, inheritance, adoption, guardianship, maintenance — was, and in large part still is, administered separately for each religious community. For Hindus it was uncodified: drawn from scripture and local custom, differing between regions and schools, and interpreted by judges working from translations of texts. Under it a woman had no right to divorce, a wife had no share in her husband's property, a daughter had no share in her father's, a widow's interest in what she held was limited and reverted on her death, and a man could take more than one wife.

The work of codifying it had begun before him. A committee under B.N. Rau was appointed in 1941 and produced a draft code by 1944. What Ambedkar did, as Law Minister from 1947, was take charge of the bill that came out of it and carry it. It would have codified the whole of Hindu personal law in one statute: monogamy, a right of divorce available to both parties on the same grounds, a daughter's share in her father's property, an absolute rather than limited estate for widows, and rights of adoption and maintenance. It was the largest single social reform proposed in independent India, and it was aimed squarely at the majority community rather than at a minority one.

The opposition was ferocious, sustained for four years, and much of it came from inside Congress. The objections were not all of one kind, and it is worth separating them. That the state had no business legislating on religion. That codifying Hindu law alone, leaving other communities' personal law untouched, was itself discrimination against Hindus — an argument still made about the same subject today. That an unelected provisional parliament had no mandate for a change of this size and should leave it to the first elected one. And, said quietly and often, that the reform was being drafted by a man who had publicly announced he would not die a Hindu.

Rajendra Prasad, the President of India, raised his own constitutional objections and let it be understood that he might decline assent — a position that would have set the head of state against the cabinet in the republic's second year. Orthodox organisations demonstrated outside Parliament. Nehru supported the bill and did not spend his authority on it at the moment it needed spending, which is the judgement Ambedkar made and which historians have argued about since.

In September 1951 the Bill was talked out. After a truncated debate, four clauses were passed and the rest was abandoned. Ambedkar resigned from the Cabinet on 27 September 1951.

When he came to explain himself in the House on 10 October, the Chair ruled that he could not make the statement at the point he had risen to make it, since a copy had not been furnished in advance; he was told he might read it later the same evening. He walked out instead, and released the text to the press. It is the only resignation statement of an Indian cabinet minister that was never delivered in the chamber it was written for.

The statement went well beyond the Bill. He set out that he had been given no portfolio of substance beyond Law, having asked for Planning and been refused; that he had been left off the Planning Commission; that the condition of the Scheduled Castes was no better than it had been under the British and that the machinery meant to improve it was not working; and that he disagreed with the government's foreign policy, including on Kashmir. It reads as the account of a man who had concluded that his presence in the Cabinet was being displayed rather than used.

He stood in the first general election in 1952, in Bombay North Central, and lost — to Narayan Sadoba Kajrolkar, a Congress candidate from his own community who had worked as his own assistant. The margin was about fourteen thousand votes in a poll of more than a quarter of a million. He had spent twenty years warning that a reserved seat inside a joint electorate returns the candidate the majority finds acceptable, and the first general election of the republic he had drafted the constitution for demonstrated it on him personally. He stood again at a by-election at Bhandara in 1954 and lost again.

He entered Parliament through the Rajya Sabha, the indirectly elected upper house, in March 1952, and stayed there for the rest of his life. The last years of his political career were spent out of office and out of favour, in poor health, and writing: on the case for reorganising India's states along linguistic lines, on the republic's unfinished business, and on the book about the Buddha he did not live to see published.