Why constitutional reform matters in this election
The story of political and constitutional reform since 2010 has mostly been one of trying and failing. After the May election, there will be more pressure to try again.
What's at stake?
On the face of it, the British constitution has come through the last five years untouched. Attempts to change the voting system, redraw parliamentary constituency boundaries and completely reform the House of Lords have all run aground.
The most dramatic challenge to the status quo was last year's referendum on Scottish independence - it was rejected by voters, but the campaign led to an unprecedented debate about how power should be distributed across the UK.
And the parliamentary system itself is coming under increasing strain, as a structure designed around two dominant political parties creaks and struggles to accommodate the rise of smaller parties.
What are the numbers?
There are 650 MPs representing the UK and more than 840 peers - five of whom have so far opted to retire under new rules introduced in 2014 to allow them to step down.
In addition, the Scottish Parliament has 129 members, the Northern Ireland Assembly has 78, and the Welsh Assembly 60.
In 2013 the number of people registered to vote for general elections was 46.1 million - and the Electoral Commission has estimated a further 7.5 million people who are eligible to vote are not correctly registered.
The 2011 referendum on getting rid of the first-past-the-post election system in favour of the alternative vote (AV) system was comprehensively won by the "No" camp, with 68% of the vote compared with 32% for the "Yes" side.
The referendum on Scottish independence was considerably closer - 55% voted against, and 45% in favour.
What won't the politicians be saying?
Whichever party - or parties - takes power in May will face demands to answer some big constitutional questions.
The parliamentary system as a whole is under pressure. There is a good chance that 2015 will see another hung parliament - prompting new questions about how the ancient machinery built to accommodate two dominant adversaries can adapt.
A significant mismatch between vote share and seats at Westminster could revive calls for an overhaul of the voting system - something any government would be reluctant to tackle after the 2011 referendum.
And even if agreement can be reached on further devolution to Holyrood (and Cardiff Bay and Stormont), the English question remains wide open.
Some commentators doubt whether it is even possible to identify which laws are "English", let alone prevent non-English MPs from voting on them. Similarly, while the Barnett formula is viewed by many as outdated and unfair, there is no clear alternative at present.
The House of Lords remains (in the words of one peer) "the Bermuda triangle of political reform" and the many parties which have pledged to replace it with an elected upper chamber will have a fight on their hands.
What has happened since 2010?
- Government has introduced a system of individual voter registration, a fixed term of five years between elections, and plans for voters to be able to recall MPs
- Limited changes to the make-up of the House of Lords, and regular debates on selected e-petitions signed by more than 100,000 people
- Referendum on Scottish independence followed by a commitment to further powers for Scotland, Wales and Northern Ireland
What do the experts say?
"The instinctive reaction of a majority of Scottish voters appears to be that their country's domestic affairs should more or less be decided in Edinburgh, leaving London just to look after its defence and foreign affairs" - Professor John Curtice, University of Strathclyde, November 2014
"If we get another hung parliament, the government will have a relatively small percentage of the vote. So if UKIP gets, say, 10% of the vote, but just a couple of parliamentary seats, its supporters will protest. PR will become a major issue after the election" - Professor Vernon Bogdanor, University of Oxford, February 2015