Both men have twice enjoyed the fruits of political power. Both have been jailed, accused of corruption, and live as millionaires in a poverty-stricken country.

Asif Ali Zardari and Nawaz Sharif, the leaders of the country's largest political parties, are now locked head-to-head in a showdown that is paralysing Pakistan.

A year ago, the pair formed an alliance that aroused hopes they would work together to roll back eight years of dictatorship. But the euphoria overlooked the historic hostility between the men.

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From 1988 to 1999, four governments headed alternately by Mr Sharif and Mr Zardari's wife, Benazir Bhutto, collapsed amid wrangling between their parties.

Mr Zardari has not forgotten that Mr Sharif twice imprisoned him. Mr Sharif believes the man he accused of corruption cannot be trusted.

Since resigning from government in August, Mr Sharif has been positioning himself as an alternative leader for Pakistan should Mr Zardari's Pakistan People's Party fall. Confronted by unrest, a souring economy and tensions with India, Mr Zardari has executed a series of self-defeating moves.

He imposed central government on Punjab province in February after a court ruled the election of Mr Sharif's brother as chief minister was invalid.

Days later, his government caved in to Taliban demands to impose Sharia in the Swat Valley.