For decades the Congress was the dominant force of Indian party politics.
But as its fortunes have declined in recent years, it has found it increasingly necessary to reach out for coalition partners.
Click on the links below to read about the Congress party's allies.
NATIONALIST CONGRESS PARTY
The Nationalist Congress Party (NCP) was formed in May 1999, when three Congress leaders Sharad Pawar, PA Sangma and Tariq Anwar broke away to form their own party.
They questioned the leadership of Congress party president Sonia Gandhi. And they said she was unsuitable to be prime minister because of her Italian origins.
But after a poor performance in the last parliamentary elections, the NCP formed an alliance with the Congress.
The party's biggest player is Sharad Pawar who is considered influential in the western Indian state of Maharashtra, where the party has a strong presence.
The NCP-Congress alliance has thrown up a mixed verdict in Maharasthra in the recent general elections.
The honours have been evenly divided between the alliance and the rival BJP-BJP coalition.
The NCP, on its part, won half of the 18 seats it contested-a mediocre performance at best.
Analysts reckon that the NCP might merge with the Congress party in near future as the two parties no longer have any differences.
The party has dropped the issue of Sonia Gandhi's foreign origins. Mrs Gandhi has also opted out of the prime minister's position.
The DMK was formed in 1949 in the southern state of Tamil Nadu as part of the Dravidian movement fighting to promote Tamil interests.
One of its key objectives was to resist the imposition of north Indian culture in the south, particularly the use of Hindi as an official language.
In 1972, a faction of the party broke away to form the AIADMK.
The DMK fought the last elections as part of the governing BJP-led alliance but has now parted ways, becoming an ally of the Congress party.
The DMK-led coalition has swept the general elections in Tamil Nadu winning all its 39 seats.
The DMK won all the 16 seats it contested.
The party will be a key and powerful ally of the Congress-led government.
RASHTRIYA JANATA DAL
The socialist Rashtriya Janata Dal is headed by Laloo Prasad Yadav and has a strong base in Bihar, one of India's poorest states.
Mr Yadav is one of India's most colourful politicians with a strong support base among lower-caste Hindus and Muslims.
Despite performing disastrously in the 1999 elections - Mr Yadav was defeated in his own parliamentary seat - the party bounced back a year later when it swept elections to Bihar's state legislature.
The party is now an ally of the Congress-led government, having performed exceedingly well in the elections in Bihar.
Leading an alliance with Congress, the RJD won 19 of the 23 seats it contested.
The Congress-RJD alliance won 26 of the 40 seats in Bihar.