Indian communists swept to power in two of five state assembly elections on Thursday, while the chief of the ruling Congress party, Sonia Gandhi, easily won a parliamentary by-election.
Congress did not have much else to cheer about in the mammoth state polls, its biggest electoral test since coming to power two years ago.
Although an ally won in a southern state and Congress won the tiny state of Pondicherry, the ruling party failed to retain its majority in the north-eastern state of Asom. Losing the southern state of Kerala to the left was considered a blow.
The state vote, however, was not being seen as a referendum on Prime Minister Manmohan Singh's two years in power as regional issues dominated campaigns in all states.
Nor were the results expected to drastically change power equations at the centre where the communists support the Congress-led coalition government.
Communist party chief Prakash Karat said the left would use its mandate to push more "pro-people measures" in government policy but would not destabilise the coalition in New Delhi.
"The election results have strengthened the role of the left in national politics," the Communist Party of India (Marxist) said in a statement.
Sonia Gandhi, the Italian-born leader of Congress, won a parliamentary by-election by a margin of more than 400,000 votes from the family borough of Rae Bareli in the northern state of Uttar Pradesh. It was roughly twice her winning margin in 2004.
Gandhi, whose husband, his mother and his grandfather have all been prime ministers, sought re-election after she quit parliament in March. The opposition had accused her of violating the constitution by being an MP as well as head of the National Advisory Council.
The communists retained power in the eastern state of West Bengal, a stronghold for three decades, and gained control of the southern coastal state of Kerala from Congress, TV stations showed.
Results from the troubled north-eastern state of Asom showed the ruling Congress would need the support of smaller groups to form a government while its ally, the Dravida Munnetra Kazhagam, won the southern state of Tamil Nadu.
Communist party workers celebrated across West Bengal, Kerala and in the capital, New Delhi.
Some activists hosted feasts in front of huge TV screens which showed live election results while others flocked to vote counting centres and applied vermilion to each other's foreheads.
The elections were held over five stages between April 3 and May 8. More than 130 million voters were eligible to cast ballots, more than the total registered electorate in Russia.