After three eventful days, the dharna launched by West Bengal Chief Minister Mamata Banerjee and cheered on by Opposition parties across the country wound up on Tuesday. In scenes that echoed the Opposition unity rally in Kolkata last month, leaders such as the Telugu Desam Party’s Chandrababu Naidu and the Rashtriya Janata Dal’s Tejashwi Yadav made speeches about saving democracy and the Constitution.

Yet the reason why national leaders had congregated under the marquee in central Kolkata was this: on Sunday evening, over 40 officers of the Central Bureau of Investigation had descended on the Kolkata police commissioner’s residence, ostensibly to question him about the Saradha and Rose Valley ponzi schemes. If the show of solidarity at the dharna was meant to be a statement of intent, the Opposition needs to be careful that it is not seen to be shielding those accused of corruption.

It is no secret that the Central Bureau of Investigation has long been deployed as a political weapon by the party in power in Delhi. Questions have been raised about the way Sunday night’s operation was conducted: why so many officers were needed for a mere interrogation, why they did not have a warrant, why previous summons to the police commissioner were not followed up on until now.

It is not clear why Bharatiya Janata Party leaders such as Himanta Biswa Sarma and Mukul Roy, also tainted by the scam, are not being investigated with the same alacrity. The timing of the inquiry, on the eve of a national election, for infractions that allegedly took place years ago, suggests that enforcing the law was not the only priority here. Sunday night’s operation did reek of political vendetta, as Banerjee alleged. It was followed up with a Bengal rally by the BJP’s Yogi Adityanath, who invoked Hindu pride and damned the Trinamool Congress for leading a corrupt, undemocratic government in the same breath.

Yet, it is true that the Saradha and Rose Valley scams, ponzi schemes that wiped out the savings of thousands of poor investors in eastern India, singed senior members of the Trinamool Congress. The central agency has charged Kolkata police commissioner Rajeev Kumar with hampering the investigation by allegedly tampering with the evidence. These are serious charges which deserve a fair investigation. As Banerjee ridicules law enforcement agencies as mere tools of the Centre, she must take care not to undermine the probes.

In the run up to the 2014 Lok Sabha elections, one of the main pitches of the BJP was that it would clean out the augean stables, break the hold of a corrupt, ossified political elite. It proved to be persuasive that year, when the United Progressive Alliance was reeling from a series scams and the coalition increasingly looked like a rogue’s gallery. Come 2019, the parties ranged against the BJP cannot afford to look like the same ragtag band of the morally and ethically compromised. If they are to be held as a principled opposition, they must show zero tolerance for corruption even as they take on the coercive tactics of the BJP.