(CNN)India's ruling Hindu nationalist party has suffered a resounding defeat in state elections, boosting the opposition before national elections next year.
Official results show Prime Minister Narendra Modi's Bharatiya Janata Party (BJP) losing the three key states of Rajasthan, Madhya Pradesh and Chhattisgarh, with the opposition center-left Congress emerging as the single largest party in each.
Modi acknowledged defeat, saying in a tweet that "We accept the people's mandate with humility." It was the BJP's first major setback in state elections since it stormed to power nationally in 2014.
Congress, which had ruled India for most of the time since independence from Britain in 1947, was relishing an unexpectedly sizeable boost before a general election which must be held by next May. Results were still coming in but it appeared set to form the government in Chhattisgarh and could lead coalitions in Rajasthan and Madhya Pradesh.
"There is a huge responsibility on Congress party now and we will work towards fulfilling promises we made in these states," party president Rahul Gandhi told a press conference as jubilant supporters danced and set off firecrackers.
"A question is being asked about the future of our youngsters and how the country will create jobs. It is pretty clear that there is a feeling throughout the country that Mr. Modi hasn't delivered what he promised."
Poll results in two other states -- Telangana and Mizoram -- showed the BJP and Congress lost to regional parties respectively.
Analysts had predicted a setback for the BJP but the results "were worse than we had anticipated" (for it), according to Washington-based think tank Eurasia Group.
The results were a sharp reverse for Modi, whose party took 282 of 543 parliamentary seats in 2014.
"The scale of anger against the BJP is quite strong and in the past, they have been able to stave it off... perhaps through campaign techniques or resorting to keeping urban voters on their side," said Neelanjan Sircar, a senior fellow at the New Delhi-based think tank Center for Policy Research.
"I think it is fair to call it a litmus test."
Modi swept into office on promises of an economic boom and a corruption-free government. However, with just months to go before the 2019 polls, his popularity has started to wane. Economic growth dropped sharply to 7.1 percent in the latest quarter, compared to the 8.2 percent GDP growth reported in August.
"The wrath seems to be amongst the agricultural population...We are now in the part of the country where there is much less urban population, urbanizing much less quickly, and therefore the anger in the rural belt -- anger among farmers -- has a much stronger impact on the electoral outcome," said Sircar.
Apart from the slowing economy, the Modi government's reputation has also been tarnished by increasing incidents of communal violence and of sexual violence against women.