Nearly six weeks after India abruptly scrapped high-denomination banknotes accounting for about 85% of its currency by value, the economy continues to slow. Poor and middle-class Indians are still suffering from the shortage of new bills. But the most significant casualty may well be Prime Minister Narendra Modi’s reputation as a sound economic manager.

On Nov. 8, Mr. Modi announced that India’s two highest-denomination currency bills—1,000 rupees ($14.84) and 500 rupees—would immediately cease to be legal tender in most places. Holders have until the end of the year to deposit these bills into bank accounts. As replacements, the government has slowly rolled out a redesigned 500-rupee bill and a new 2,000-rupee bill.

The long-term effects of India’s demonetization gambit remain unclear, largely because no other major economy has attempted such an experiment except during a crisis. But with growth slowing and job losses rising, the short-term prognosis appears grim.

Instead of factory openings or large new investments, the images that tell India’s current economic story include snaking lines outside banks, distressed workers migrating back to their villages, and tax raids on jewelers and officials caught with hoards of allegedly illicit cash.

The policy’s shabby implementation—through an avalanche of amendments, rollbacks and patchwork fixes—undercuts Mr. Modi’s reputation for quiet efficiency. It also underscores his overreliance on India’s notoriously heavy-handed bureaucrats.