But this week in the Commons, only one of these parties - the Conservatives - will honour that pledge by voting for the Lisbon Treaty to be put to a vote of all the British people before it becomes law.
No wonder trust in politicians is at an all-time low.
Labour and the Liberal Democrats have behaved shoddily in reneging on their solemn, written promise of a referendum.
Blithely ignoring the fact that the Lisbon Treaty is the European constitution in all but name - just look at the 60 national vetoes it scraps - Gordon Brown and Nick Clegg say that a referendum is now unnecessary.
But if Brown is treating the British public with contempt, Clegg must think they are fools.
In an attempt to disguise his party's betrayal of the voters, Clegg has come up with a new wheeze - a referendum not on the Treaty, which he knows he would lose, but a referendum on whether Britain should stay in Europe, which he calculates he would win.
Clegg is being too clever by half. He promised a referendum on the Treaty that is now on the table. He did not promise a referendum on something else entirely.
But he looks like coming a cropper. A quarter of his frontbench team of spokesmen are threatening to rebel and vote for the Treaty referendum despite his double-dealing.
The new LibDem leader is swiftly demonstrating continued incoherence, a political trait that has disqualified his party as a serious party of opposition for the past 100 years.
