By Sanjoy Majumder BBC News Online correspondent in Delhi

Congress holds power in all four contested states

The BJP is trying to unseat the Congress party in Rajasthan, Madhya Pradesh, Chattisgarh and the national capital, Delhi - results are due on Thursday.

Congress, led by Sonia Gandhi, is the country's main opposition having ruled India for five decades.

It is aiming to hold the states to boost its challenge against the BJP in national elections next year.

For Prime Minister Atal Behari Vajpayee, the polls represent an opportunity to test the waters ahead of those polls.

Although local issues are the focus in the states, a strong showing for the BJP could prompt him to call a general election a few months ahead of schedule, analysts say.

Congress, for its part, believes a good result will indicate that it has governed the states well, thus persuading voters to give it a fresh chance at federal level.

Hindi heartland

The four states voting on Monday all form part of the politically influential Hindi-speaking belt in north and central India, often described as the Hindi heartland.

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Together with the states of Uttar Pradesh and Bihar they send more MPs to parliament than the rest of India put together.

A good showing here can propel a party to power and most Indian prime ministers have been elected from this region.

This also means that regional leaders who are powerful have a chance to extend their career to the national stage.

All four states are witnessing a straight fight between the Congress and the BJP, with the influence of smaller parties marginal at best.

This is a shift from the otherwise multi-party character of recent Indian elections, with the present national government being made up of a coalition of more than 20 parties.

Key players

Several charismatic and young leaders are leading the two parties in these polls and a strong showing will help increase their political stature and influence.

In Madhya Pradesh, Chief Minister Digvijay Singh is hoping to be returned to office for a third straight term.

The media-savvy Mr Singh, who is from an aristocratic background, is perceived to be reformist and many believe has a future in national politics.

But he faces a stiff challenge from the BJP's Uma Bharati, a Hindu nun who is a charismatic leader of the Hindu right.

In the desert state of Rajasthan, the Congress's Ashok Gehlot is being opposed by Vasundhara Raje of the BJP.

Ms Raje belongs to one of the country's most noted royal families, the Scindias, with ties to the Nepalese royal family.

In the national capital, Delhi, Chief Minister Sheila Dixit is hoping to be returned to power on the back of the city's improved transport system and pollution levels.

While both Ms Dixit and her BJP opponent, Madan Lal Khurana, are taking credit for improving the capital, they blame each other for Delhi's worsening security situation, particularly after an increase in crimes against women.

National scene

A strong showing by the Congress will strengthen Ms Gandhi while placing the party in pole position ahead of the general elections.

The BJP, on the other hand, may well decide to bring those polls forward to cash in on its appeal if it does well.