The Bharatiya Janata Party has enjoyed a rapid rise to the top since winning its first parliamentary seats in the 1980s.
The core of its support is in the Hindi-speaking belt that cuts across northern India.
Although making inroads into other parts of the country, the BJP is still heavily dependent on alliances with other parties.
Click on the links below to read about the BJP's allies.
JANATA DAL (UNITED)
This socialist party is one of the many manifestations of the erstwhile Janata Dal (People's Party), which was formed in 1989 and formed a short-lived government after defeating the then Congress Prime Minister Rajiv Gandhi.
In its latest incarnation, the Janata Dal (United) merged with another regional party, the Samata.
It is headed by the veteran socialist leader and the former Defence Minister, George Fernandes.
The Janata Dal (United) was a powerful member of the previous National Democratic Alliance (NDA) coalition with 20 seats in the outgoing parliament after a strong performance in Bihar in the last elections.
But it suffered a major setback in these elections winning only eight seats.
Mainly made up of socialist politicians from Bihar, several party heavyweights including three cabinet ministers lost.
The Telugu Desam party is based in the southern state of Andhra Pradesh.
It is led by Chandrababu Naidu and was the most powerful of the BJP's partners in the former NDA governing coalition with 29 seats.
But it was trounced in both the state and parliamentary elections, winning only five seats in the lower house.
The Telugu Desam was founded in 1985 by a former Telugu-language movie star, NT Rama Rao.
Mr Naidu became its undisputed leader after leading the party to an impressive victory in elections to the state legislature which were held along with the 1999 national elections.
Mr Naidu is one of India's new generation leaders. His image as a dynamic reformer is largely based on the development of the Andhra Pradesh state capital, Hyderabad, into one of India's information technology hubs.
But his defeat in these elections has cast him in the political wilderness, at least for the moment.
He is now looking to lead his party to victory to a third successive term while continuing to back the BJP at the federal level.
Shiv Sena is an ultra-nationalist Hindu party based in the western state of Maharashtra with a powerful presence in the state capital Bombay.
It is headed by one of India's most controversial and militant right-wing leaders, Bal Thackeray.
He formed the Shiv Sena in 1966 as a "sons of the soil" movement, to fight for the rights of native Maharashtrians who, the party alleged, were under threat from other ethnic migrants.
The party ran an intimidating and often violent campaign, targeting southern Indians who worked as clerks or owned small restaurants in Bombay.
In later years, the Gujarati and the Muslim communities were similarly targeted.
Over the years, the party has acquired a reputation of promoting religious and ethnic chauvinism while targeting minorities, especially Muslims.
An important ally of the BJP, the western state of Maharashtra remains the Shiv Sena's main support base where it formed its first government in 1995.
The AIADMK is based in the southern state of Tamil Nadu and has its roots in the Dravidian movement promoting Tamil identity.
It was formed in 1972 after breaking away from the DMK - the main Dravidian party in Tamil Nadu and now the AIADMK's principle regional rival.
Led by the mercurial Jayalalitha, the party once again allied with the BJP for the 2004 elections despite bitter previous fall-outs.
Part of the BJP-led ruling coalition government in 1998, it pulled out in 1999 - an action that caused the government to fall and hold snap elections.
But the BJP could well be regretting its move to partner the AIADMK in 2004 since the alliance failed to win a single seat in Tamil Nadu.
A Sikh party, it is a major player in the northern state of Punjab where it is currently in opposition.
The Akali Dal was formed in 1920 to represent Sikh interests.
In the troubled 1980s radical elements from within the Akali Dal pushed for the creation of an independent Sikh state, a demand that has since been dropped following the Punjab peace accord of 1985.
The Akali Dal formed a formidable alliance with the BJP in 1997 which has endured despite the combination losing the Punjab state elections in 2002.
It led to a strong performance in the 2004 elections with the alliance winning 10 out of 13 seats in Punjab.