Dropping the biggest bombshell to date in Chinese President Xi Jinping’s anticorruption campaign, the ruling Communist Party announced Tuesday that one of its most senior elder statesmen is being investigated for suspected “serious disciplinary violation.”

Zhou Yongkang, who was head of China’s all-powerful security apparatus and one of the party’s top nine leaders until 2012, has not been seen in public for months. If tried and convicted he would be the most senior official found guilty of corruption since the founding of the People’s Republic in 1949.

The news of his detention had long been anticipated. Since Mr. Zhou disappeared at the end of last year, reportedly held under house arrest, scores of his relatives, former aides, and political allies have been detained in a campaign to destroy his political support network.

For the past 18 months Mr. Xi has been orchestrating a crusade against the widespread corruption that afflicts the ruling party, pledging to bring down both “tigers” – powerful figures – and “flies,” more junior officials. Zhou is the fiercest tiger yet captured in the hunt.

The official announcement that he is under investigation – which in China is tantamount to a declaration of his guilt – appears to mark a new victory in Xi’s bid to stamp his personal authority on the Communist party leadership and to eliminate or cow actual or potential rivals.

It is also likely to fuel debate inside and outside China about whether the anticorruption campaign – only the latest, albeit the most wide-ranging in a long history of such efforts – is a political bid to consolidate Xi’s position or a real drive to clean up the Communist Party.

There are good reasons to believe that it is both.

Of course the campaign is political. It is not the police force, nor any prosecutor, who is choosing the targets for investigation nor gathering the evidence against them. It is the Central Commission for Discipline Inspection, an organ of the Communist Party. Xi and his ally Wang Qishan, head of the CCDI, decide, for their own reasons, whom to bring down and when.

And in a system where corruption is endemic, reaching to the inner sanctum of state power to judge by some of the cases that have come to light, Xi and his closest confidants also decide who will be spared. Until a close friend or political ally of Xi’s is investigated on suspicion of corruption, the campaign will inevitably carry the whiff of political motivation.

Nor can the campaign continue at its current intensity forever, without destabilizing the networks of power and influence that hold the Chinese system of government together. When to ease off will also be a political decision.

But the campaign is clearly more than a high level assault on senior figures who might in some fashion threaten Xi’s personal position.

The president seems to see his high-risk crusade as nothing less than a battle for the Communist Party’s survival, essential to improving its legitimacy in the eyes of ordinary citizens. Chinese leaders have been warning for years that blatant official corruption is eroding such faith as people have in their rulers. None of them, until now, has done much about it.

So far, about 30 senior officials, ranking at or above vice minister level, have been detained, put under investigation or expelled from the party for “serious disciplinary violations,” officialese for bribery. Until today the most prominent was Gen. Xu Caihou, a retired military man who had been the second-highest ranking officer in the People’s Liberation Army.

But in just the first five months of this year, nearly 63,000 lower ranking officials had also been disciplined according to figures published in the People’s Daily, the Communist Party’s mouthpiece, up 35 percent from the same period in 2013.

It is by no means clear, however, that even the most sincere campaign by the most honest officials could ever eradicate the bribery and corruption that permeates the Chinese Communist Party. One four year study of a county administration in central China by Graeme Smith, an Australian academic, found that 80 percent of government posts there were bought and sold. Dr. Smith chose to study that county because it was typical.

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There is an aphorism attributed to Chiang Kai-shek, Mao Zedong’s great rival and head of the Guomindang party during the civil war that led to the Communists’ victory. “Corruption has gone deep into the bones of the Guomindang,” he is said to have commented. “Combat it and the party will die. Do not combat it and the nation will die.”

Chinese audiences who heard the Chiang Kai-shek character utter those words in a recent historical film laughed and clapped. They recognize the contemporary dilemma that the nationalist leader’s successors now face.