The 50 constituencies fall mainly in Murshidabad, Nadia and Birbhum districts, which will go to polls April 23.

The last date for filing nomination papers is April 6. The scrutiny of the nominations will be held April 7 and the last date for the withdrawal of candidature will be April 9.

The Murshidabad district with 22 assembly constituencies is considered a Congress citadel. The ruling Left Front has a few pockets of influence in the district.

The Nadia district with 17 assembly constituencies has a strong presence of the Left Front as well as the Trinamool Congress, with a few pockets of the Congress. In 2006 polls, the Left Front won most of the seats.

The Birbhum district, home to Rabindranath Tagore's Santiniketan, has 11 assembly constituencies and is considered an impregnable fort of the Left Front.

All eyes will be on the Nalhati assembly constituency in the district where union Finance Minister Pranab Mukherjee's son and Congress candidate Avijit Mukherjee will fight Left Front candidate Dipak Chatterjee.

The Trinamool Congress and the Congress, along with the Nationalist Congress Party (NCP) and the Socialist Unity Centre of India-Communist (SUCI-C), will jointly fight the Communist Party of India-Marxist (CPI-M) led Left Front which has been ruling the state uninterruptedly since 1977.

The SUCI-C has declared that in some places it will field candidates against the Congress.

During the 2009 Lok Sabha polls, the Trinamool Congress, the Congress and the SUCI-C fought together against the state's ruling Left Front and bagged 26 out of 42 Lok Sabha seats in the state.