NEW DELHI  The governing coalition led by the Indian National Congress sailed to a surprisingly decisive victory in India’s grueling parliamentary elections, vaulting Manmohan Singh, a soft-spoken economic reformer, to a second term as prime minister, and sweeping away the prospect of political instability in the world’s most populous democracy.

Mr. Singh, 77, called the victory “a massive mandate” on Saturday afternoon, hours after the opposition Bharatiya Janata Party conceded defeat. The victory, in what is apparently a Congress landslide, signals the possibility of a stable and strong government in the face of stiff challenges: a sharp slowdown in economic growth, abiding poverty and instability in the region.

It also sidelines a slew of small, regional party bosses whose influence had steadily grown in Indian national politics, and potentially cuts down the power of Communists who had blocked economic reforms for most of Mr. Singh’s first five-year term.

The Congress Party’s showing vindicates the prime minister’s efforts to deepen a strategic partnership with the United States at a time when the Obama administration is deeply concerned about security in the region, chiefly in Pakistan and Afghanistan. A stronger government will also be better able to tackle issues of crucial importance to Washington, from economic reforms to climate change, although there is not necessarily agreement with the Americans on how to proceed.