Make it law that any victorious party has to fulfill all manifesto pledges made during an election campaign. If it is unable to implement a manifesto pledge, the government must supply a valid reason why that manifesto pledge cannot be fulfilled.
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Under the current rules, there is nothing to prevent a party saying what ever it wants in order to get power, then going back on it or doing totally the opposite once elected. There needs to be legislation that prevents any party saying 'what the public wants to hear' unless it has an obligation to do what it says. No political party should be allowed to just make things up for its own interests.