Here are some questions and answers on the bill which was passed in the upper house on Tuesday, but still needs the approval of the lower house.
The bill would reserve one-third of seats in the directly elected lower house of parliament and the state assemblies for women. There are 59 women lawmakers in the lower house of parliament at present, out of a maximum of 545.
Championed both by Prime Minister Manmohan Singh and Congress chief Sonia Gandhi, the legislation aims to help empower women politically and thus economically in a country where they lag far behind on many social and health indicators.
While parliament is mostly populated by older men, India has a history of women at the top of the political class, including Sonia Gandhi and her mother-in-law, the former Prime Minister Indira Gandhi.
But millions of poor women face steep odds, with shorter schooling, worse nutrition and a literacy rate more than 20 percentage points lower than men.
A row over the bill has been brewing for years, having first been introduced as far back as 1996.
Though it enjoys widespread support, underlined by a near unanimous vote it won in the upper house of parliament, some regional parties oppose the bill arguing it would move at the expense of Muslims and other minorities.
The Samajwadi Party (SP) and Rashtriya Janata Dal (RJD), whose vote bank is mostly made up of such minorities, have vowed to fight the bill "tooth and nail" once it is introduced in the lower house of parliament.
For an online debate on the bill, go to http://blogs.reuters.com/india/2010/03/08/some-questions-on-the- womens-reservation-bill/
Though it still has a majority in parliament, Congress could lose a chunk of political capital it needs to push through key economic legislation and deal with a slew of setbacks since winning a second term last year.
The Samajwadi Party and the Rashtriya Janata Dal have withdrawn their outside support to Manmohan Singh's coalition, costing the government 26 seats in the lower house.
The Trinamool Congress party, a key coalition partner whose leader is the railways minister, boycotted Tuesday's vote, saying Congress had not properly consulted its ally on strategy.
Trinamool has threatened to do the same in the lower house, which could pile on trouble for Congress already bracing for more protests against the bill in the second round of voting.
Meanwhile, the Hindu-nationalist Bharatiya Janata Party, trying to make a comeback after a heavy election defeat, has played up what it says was Congress' poor handling of rowdy protests in parliament that saw seven lawmakers suspended. The party supported the bill.
The standoff comes at a bad time for Congress. Though the economy is expected to grow more than 8 percent in the next fiscal year, the government is grappling with a 16-year-high deficit that is forcing it to take unpopular decisions.
Congress is under fire over food inflation, a proposed hike in fuel prices, and the creation of a separate state in southern India that continues to set off violent clashes and suicides.
With two allies gone and a third dissenting, Congress might find itself having to pander to coalition allies, or reach out to unreliable third parties.
An unwieldy alliance or a protracted fallout from the bill may also further delay debate on bills such as on land acquisition and entry of private players into the pension sector.
Though highly unlikely, some commentators even go as far as to say the government could face hurdles pushing through the 2010-2011 budget, which was announced in February.
Again, highly unlikely. Congress leaders have so far played down the standoff, saying the coalition is intact, and Trinamool has not threatened to split from the government.
The two parties need each other to fight the state election next year in the state of West Bengal, which is Trinamool's stronghold and where the allies want to wrest control away from the ruling Communist party.
Both the SP and the RJD have so far said they will not try to bring a "no-confidence" motion against the government, saying they do not have the numbers to do so.
