There is no more fascinating organism than politics, that abstraction of the human desire for power. It is a game in a closed system where actions result in outcomes that close or open the doors in the hallways of power.It is not an organism that likes to show itself. If forced into the open, it prefers the disguise of a noble agenda. But there are times it can no longer hide, when its actors must show themselves, and take an opportunity that presents itself.The budget speech by Finance Minister Pravin Gordhan was such an occasion: the opportunity to make a decisive break from the Zuma era's policy muddle began to present itself in December, when Zuma, wearing a growing burden of scandal, pushed the wrong button. His calculation was that the National Treasury, one of the few bastions of rationality, was ripe for the picking.He struck with speed and surprise.Late one evening, after a cabinet meeting in which he had said nothing of what was to come, he fired the finance minister, Nhlanhla Nene, replacing him with David van Rooyen - an unknown backbencher whose only moment in the spotlight was when he had tried to milk the system with a dairy farm deal involving the Gupta family.Zuma, who had scaled the Everest of politics when he came back from being fired by Thabo Mbeki to depose him in the beer-hall putsch at the ANC's 2007 conference, was a canny operator. That he made this mistake was testimony to the political pressure he was under.By the time Van Rooyen had wheeled his briefcase to a lesser ministry and Gordhan had returned as finance minister, the breadth of Zuma's miscalculation was clear to all. An opportunity had presented itself, and it had been taken.Zuma's miscalculation was to underestimate the value attached to the Treasury's reputation for independence and professionalism.It was not just another ministry. The public - rich and poor - knew it was the last thin line between a functional and a failed state, a seldom-articulated perception powerful enough to overwhelm Zuma.By appointing Gordhan, a man he had removed from the Treasury in 2014, Zuma had acknowledged that power resided somewhere else. Once this occurred, it was inevitable that Zuma would capitulate in his other major battle: the fight to reduce the public protector's authority.His counsel told the Constitutional Court that Zuma would, despite years of prevarication, pay back the money owed for his home improvements to Nkandla that were not related to security. In so doing, Zuma weakened himself by slashing through the credibility of a phalanx of politicians who had gone out on a limb to defend him. They were like puppets dropped helplessly onto the stage by a distracted puppet master.Gordhan's speech on Wednesday was remarkable for its bold reassertion of fiscal discipline and its attempt to send a signal that South Africa would return to sound economic fundamentals. There were three moments that underlined the shift in political fortunes:The first occurred when Gordhan mentioned his predecessor, Van Rooyen. There was loud jeering in the house, and the ANC benches, which usually come to the defence of one of their own, did not rise to defend Van Rooyen. Rising in defence of Zuma had lost its appeal.The final two moments occurred when Gordhan thanked those who had contributed to his speech.He added a line not in the text and thanked his other predecessor, Nene, for his service. To this, there was loud and sustained applause.It was a moment of public shaming for Zuma, who sat impassively. Then Gordhan thanked the members of the South African Revenue Service. More loud applause: Gordhan is in a battle with SARS head Tom Moyane over plans to "restructure" it. The applause was the House taking sides.The budget speech was a political stake in the ground, one which Zuma is powerless to move. What must surely follow is the expansion of the Gordhan moment into a total displacement of Zuma and his allies.Your move, Mr President.Hartley is Times Media editor at large and author of 'Ragged Glory: The Rainbow Nation in Black and White'..