The alliance of Sri Lankan President Chandrika Kumaratunga says it will act to scrap the presidency if it wins the general election on 2 April.
President Kumaratunga cannot run for a third term in office
The United People's Freedom Alliance manifesto wants a new constitution to restore full parliamentary democracy.
The president and prime minister, who are both elected, have been locked in a long battle for power.
Mrs Kumaratunga cannot run a third time for the presidency, but she could serve as prime minister.
Her term in office is scheduled to end in 2005, although she argued recently that events surrounding her last election meant she could remain in office until 2006.
She has fought a long battle for power with Prime Minister Ranil Wickramasinghe whom she accuses of giving too much ground to Tamil Tiger rebels in the country's peace process.
The 48-page alliance manifesto, released late on Thursday, said: "The proposed constitution will strengthen democracy by abolishing the executive presidency and replacing it with a cabinet and parliamentary form of government."
The alliance said the new constitution would be put to the Sri Lankan people in a referendum.
Mr Wickramasinghe's party brought in the presidency in 1977
Mr Wickramasinghe has yet to respond to the proposal.
It was his United National Party that altered the parliamentary democracy in 1977 by introducing the presidency.
The president can sack the government and call elections at any time and is head of the armed forces.
Mrs Kumaratunga angered the prime minister's party in January by announcing she was entitled to another year in office.
She argued that her first term in office had been cut short by early elections and that there had been a second, secret swearing-in ceremony to extend her term until the end of 2006.
Her power struggle with Mr Wickramasinghe came to a head last November when she took over three of his top ministries.
She dissolved parliament last month and called the April polls.
International monitors of Sri Lanka's peace process have said they cannot continue their work because it is unclear who is running the country.
The manifesto of the alliance - which groups Mrs Kumaratunga's People's Alliance with the Marxist JVP, also sets out plans for more subsidies in the state sector.
"The railways, cluster bus companies, petroleum, electricity, ports and airports, water and state banks will not be privatised,"