At the session this evening, Mr Crosby - an Australian who masterminded London Mayor Boris Johnson's re-election - was said to have reminded MPs that they were "participants not commentators".
The politicians were urged to focus on issues where the Conservatives polled strongly against Labour, such as welfare reform and the promise of an in-out referendum on EU membership. They were also told to stress that the next election would be a choice between having Mr Cameron or Ed Miliband in Downing Street.
Most of the interventions at the private meeting were said to be positive and supportive of the leadership. However, Totnes MP Sarah Wollaston apparently pointed out that Mr Crosby's call for discipline had been leaked to the media beforehand - asking whether the rules applied equally to the top of the party.
Others complained that Cabinet ministers did not seem to be bound by the same restrictions, while Keighly MP Kris Hopkins reportedly criticised "self indulgent buffoons" touting themselves as alternative leaders.
It was reported that Education Secretary Michael Gove had made a thinly-veiled dig at Mrs May during a political Cabinet session on Tuesday morning. Without naming the Home Secretary, Mr Gove apparently warned that senior figures who promoted their leadership credentials were playing into the hands of the Tories' opponents.