India's ruling Bharatiya Janata Party on Tuesday won a by-election in a troubled corner of the country, after running a divisive campaign that opponents called a test case for a bigger state election in 2017. Communal tension remains high in Muzaffarnagar in northern Uttar Pradesh, India's most populous state, three years after 65 people were killed in clashes between Hindus and Muslims, the worst bloodletting in the area in decades. Prime Minister Narendra Modi's BJP swept the Muzaffarnagar election, beating out powerful regional parties and reducing its main nationwide rival, the Congress party, to third place, the results released by the election commission showed. In the run-up to the vote, state BJP leaders sought to rally voters behind their Hindutva campaign, or the idea that today's multifaith India is fundamentally a Hindu nation. Critics see the campaign as an attempt to drive a wedge between Hindus, who make up 80 percent of India's 1.3 billion people and Muslims, Christians and others. Next year, the whole of Uttar Pradesh elects a new state assembly in what will be Modi's most important test of popularity before he seeks a new term in 2019. The Congress party, which has been trying to regain ground since its defeat in the 2014 national election, won Deoband in another part of the state, which is home to a leading school of Islamic thought and a large number of Muslims.