Jinnah sensed well before the end of February 1928 that Hindu Mahasabha pressure had persuaded Congress to back off from its acceptance the previous May of his new constitutional compromise.To see what happened, we need to see what happened in the Indian National Congress meeting, in December 1927. Subsequents posts will do the needful.
PS: The Congress, in its Madras, December 1927 session, carried a resolution unanimously that seems to have accepted the Delhi Muslim Proposals. I'll post the proceedings later. The question now is what happened after December 1927 but before February 1928 for Jinnah to feel that the Congress had backed away from the proposal?
PPS: What Wolpert does tell us is that the Muslim League had a split and held parallel sessions in Calcutta and Lahore. What Wolpert does not tell us is that while the Calcutta session, attended by Jinnah, reaffirmed the Delhi Muslim Proposals (except that Sind, Baluchistan, N.W.F.P. reorganization had to be completed before joint electorates with reservations would come into effect), the Lahore session rejected the proposal.
This meeting of the All-India Muslim League declares that neither the proposals formulated by some Muslims in their individual capacity at Delhi on March 2nd 1927 {sic, should be March 20th 1927}, in their original form nor in their amended form as passed by the Congress at Madras are acceptable to the Mussalmans of India.The next step in this examination will be the All Parties Conference in Delhi, February 12-22, 1928.
After that, Wolpert tells us that in March 1928,
Jinnah convened his League council, which officially "regretted that the Hindu Mahasabha has practically rejected the Muslim League proposals."IMO, this is a way of backing out that Jinnah used later: when he inadvertently came to accord with Rajendra Prasad in 1935, he then demanded that the Hindu Mahasabha leaders must accept the accord, and that was the end of that. Of course, that is my guess - let us see what happened as recorded in the Indian Annual or Quarterly Register.