Mayawati has emerged as a key centre of power
India's communists and several other opposition parties have launched a joint campaign over rising prices and a controversial nuclear deal with the US.
The new group contains four left-wing former allies of the government and a Dalit leader tipped as a future PM.
They say the government is tainted by corruption claims even though it won a confidence vote over the nuclear deal.
The communists also expelled Speaker Somnath Chatterjee, a party veteran, for not resigning ahead of the vote.
The confidence vote came after left-wing parties withdrew support from the government in protest at the nuclear deal.
If the government had lost the vote, India would have faced early elections and the nuclear deal would have been in doubt.
Correspondents say the confidence vote debate has been used by many politicians to kick off campaigns for general elections due in the first half of next year.
"All the political parties present decided to launch a joint national level campaign on pressing issues before the people," Prakash Karat, general secretary of the Communist Party of India (Marxist) or CPI(M), told a press conference in the capital, Delhi.
Mr Singh called the vote an "impressive victory"
The new group says it will campaign on the issues of inflation, the India-US nuclear deal and farmer suicides.
It consists of four left-wing parties - all former allies of the governing Congress party - and six other regional parties. Together they are believed to account for nearly 100 MPs in India's 543-seat parliament.
The most prominent among the regional groups is the Bahujan Samaj Party (BSP) led by Mayawati, chief minister of India's most populous and politically influential state, Uttar Pradesh.
Ms Mayawati is a Dalit (formerly "untouchable" in the Hindu caste system) icon who openly talks about her ambition to become prime minister soon.
In the last few days of hectic political activity several new alignments have been formed. Ms Mayawati has emerged as one of the most important political leaders to watch in the days to come.
"The way this government has been saved, it is a big loss for democracy," she said in her first reaction after the confidence vote.
Communists expel Speaker
Amid frantic horse-trading, MPs were summoned from their sick beds and even from prison cells to take part in the vote.
Mr Chatterjee stays on as Speaker
A two-day debate on the nuclear accord which preceded the vote ended in uproar amid opposition allegations of vote buying.
The vote initially looked too close to call, but the government won with a wider margin of victory than many predicted, thanks to the support of smaller parties and independent members.
On Wednesday, the CPI-M said Speaker Somnath Chatterjee had been expelled with immediate effect "for seriously compromising the position of the party".
Communist leaders had wanted him to resign before the vote in order to be free to vote with the party.
The main opposition Bharatiya Janata Party (BJP), which made the vote-buying claims now being investigated, also expelled eight of its members for not obeying the party whip during the vote.