MUMBAI, India — A jeans maker saw his delivery costs cut by half when the highway police stopped asking for bribes. An aluminum wire factory faced only three inspectors rather than 12 to keep its licenses. Big companies like Corning, the American fiber-optic cable business , found they could wield a new bankruptcy law to demand that customers pay overdue bills.

Prime Minister Narendra Modi promised nearly five years ago to open India for business. Fitfully and sometimes painfully, his government has streamlined regulations, winnowed a famously antiquated bureaucracy and tackled corruption and tax evasion.

But cutting red tape has yet to translate into broad growth for this emerging economy, or spark much outside investment. Small and medium-size businesses have struggled to keep up with the pace of the overhaul. Some, which seldom paid taxes before, went bankrupt. Civil servants stumbled repeatedly in efforts to turn Mr. Modi’s abruptly issued policies into simple-to-follow standards.

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Now, as 900 million Indians go to the polls, Mr. Modi has to persuade voters to stay the course and convince them that his reforms — unconventional to many — are taking root. As the campaign between his Bharatiya Janata Party and the opposition Congress Party heats up, Mr. Modi faces an opponent, Rahul Gandhi, who contends that the prime minister has disrupted a functioning economy and caused job losses.