NEW DELHI — Prime Minister Narendra Modi of India swept into office four years ago vowing to crush corruption. Some of his most momentous acts since taking office, like wiping out nearly 90 percent of India’s currency virtually overnight, have been to clean up the country’s endemic graft.

Mr. Modi was also quick to accuse the rival Congress Party, India’s longstanding political dynasty, of lavish spending and crony capitalism.

But in the past few weeks, the role of accuser has been dramatically reversed.

The Congress Party, which had seemed anemic since Mr. Modi’s election, has found a new spring in its step by hounding the prime minister over an opaque arms deal that has raised some serious questions.

Why did Mr. Modi renegotiate a deal for 36 fighter jets?

Why was a company run by members of one of India’s wealthiest families chosen to participate in the deal, despite having no experience building jets?