By Jill McGivering BBC South Asia correspondent

Four states go to the polls in India this month - but the main focus is on the battle for control of Uttar Pradesh.

If it [the BJP] were to lose outright control of UP, it would be a danger signal for the party as a whole

With a population of more than 160 million people, it is India's most populous state, one of its poorest and often seen as a litmus test of national political popularity.

For the past five years, the BJP has controlled the state, albeit by a narrow margin.

But it faces a tough challenge, especially from the strong regional Samajwadi Party which is countering the BJP's association with a Hindu nationalist agenda by appealing to Muslim voters, as well as those from lower-caste Hindus.

BJP's worry

As the incumbent, the BJP may be damaged by the general dissatisfaction in a state which is fraught with poverty and social problems.

Voters are concerned about lack of development

Voters' concerns range from bad roads and the rising price of basic commodities to unemployment and high rates of crime.

Many feel they have seen little evidence of positive development in recent years to address their concerns. The BJP's reputation has been further tarnished by allegations of poor governance and constant party in-fighting.

The appointment of the present Chief Minister, Rajnath Singh, seen by many as more sincere and dynamic than his predecessors, may have restored some confidence but probably not all.

Vajpayee

The BJP is well aware how much is at stake. If it were to lose outright control of UP, it would be a danger signal for the party as a whole.

It could seriously damage the stability of the fragile national coalition, led by the BJP, if wavering coalition allies felt under pressure to change allegiances to keep in line with public opinion.

Vajpayee is under pressure from the Hindu right

A defeat could also weaken the personal authority of the BJP Prime Minister, Atal Behari Vajpayee.

His reputation as a moderate statesman has helped to hold the coalition together but also attracted criticism from more hard-line elements within his own party's ranks.

Those hardliners draw strong support from the tide of Hindu nationalism which is a major political force within UP.

Campaigning here can be crude and intense.

Many politicians try to secure voters, many of them illiterate villagers, by appealing to caste loyalties and through emotive rallying cries, often invoking nationalism.

It's a dangerous formula which makes the election process all the more volatile and, in the past, often violent too.

This too puts Prime Minister Vajpayee under strain.

As well as maintaining his own carefully nurtured image as a moderate, he must at the same time be a champion of the state's dominant Hindu right.

Critics have accused him of nurturing, as a show of pre-election strength, the current stand-off between India and Pakistan.

Certainly his noticeably tough anti-Pakistan rhetoric on the campaign trail may help to consolidate right-wing support.

Ayodhya

More problematic for him is the controversial cry from the right wing Hindu group, the Vishwa Hindu Parishad (VHP) for the building of a Hindu temple in Ayodhya, where the destruction of the Babri Mosque in 1992 by Hindu extremists sparked some of India's worst Hindu-Muslim violence.

Voting in UP

First round: 14 February Second round: 18 February Third round: 21 February

The site is now subject to the courts - but it's an explosively emotive issue which hard-liners see as a test of their Prime Minister's loyalty to the Hindu right's cause.

UP has become notorious for allegations of vote rigging, voter intimidation, campaign-related violence and political corruption.

For large numbers of impoverished, illiterate villagers, the elections are not about democracy in action nor any hope of bringing actual change but much more about an excuse to enjoy the colourful spectacle of political rallies which attract tens of thousands of people and constitute a rare form of free public entertainment.

Some of the country's biggest film stars, including Bollywood legend Amitabh Bachchan, have taken to the hustings, singing and dancing to vast crowds.

The result is still far from certain. Many are now predicting a hung assembly, with the BJP and Samajwadi Party dominating but each failing to win an outright victory.

But with so much volatility within the electorate, the only real answers will come with the results themselves. And they could be critical.

One political saying being much repeated here is - whoever controls Uttar Pradesh, controls Delhi too.