Do party leaders really win elections?
The idea that party leaders are decisively important in the winning or losing of general elections is implicit in much political journalism and it is a belief that some political leaders themselves - Tony Blair, in particular - have been eager to propagate.
It is very rarely true.
It is only in an extremely close-run race that the personality of the leader and the gulf between that leader's standing and the popularity of his or her principal opponent can make the difference between victory and defeat.
It is not even particularly uncommon for the political party of the less popular leader of the two main parties to be the one that wins the election.
Thus, for example, although journalists still write of "Margaret Thatcher's rout of James Callaghan", the Labour leader was some 20 points ahead of Mrs Thatcher on the eve of the Conservative victory in the 1979 election.
It was not Thatcher who defeated Callaghan but the Conservative Party that defeated Labour.
A serious study of post-World War Two UK elections found that the only leader who could have made the difference between his party forming a government or being in opposition was Harold Wilson, and on two occasions - in October 1964 and February 1974.
That was because those elections were extremely close and Wilson was vastly more popular than the Conservative leader in each case - Sir Alec Douglas-Home in 1964 and Edward Heath in 1974.
In between, the Conservatives had won the 1970 election, notwithstanding the fact that then, too, Wilson was more popular than Heath.
Tight contest likely
Since all the available evidence suggests that the May 2015 election is likely to be a cliff-hanger, with a distinct possibility that once again no one party will have an overall majority, does this mean that journalists' excessive focus on the top leader might for once be justified? Probably not.
The two main political parties would be well advised to give ample interview time to other front-benchers rather than over-expose David Cameron and Ed Miliband in what, thanks to the fixed election date, is going to be a very long campaign by British standards - four whole months.
Nick Clegg might also cut his losses by sharing (with Vince Cable in the first instance) whatever diminished limelight the Liberal Democrats will secure.
Mr Clegg is highly unlikely to emulate his 2010 campaign, when he was deemed the outstanding performer, especially in the televised debates, of the three party leaders.
Even then, with unprecedented mass media exposure and "Cleggmania" rampant, the party's national vote rose by only 1%, and the Lib Dems ended up with five fewer MPs than in 2005.
Nigel Farage will doubtless continue to dominate the UKIP coverage, although the defection, and subsequent election under the UKIP banner, of two Conservative MPs makes him appear less of a one-man band than hitherto.
The rise of UKIP, however, is not because it has a leader of exceptional ability - he has the gift of the gab but we don't know, and may never know, if he would make a good minister.
Their by-election and opinion poll success is principally because the party is the respectable face of anti-immigration sentiment.
At a time when the major parties fought shy of the issue, UKIP articulated widespread popular concern about high levels of immigration.
Their challenge to British membership of the European Union is a less salient preoccupation for most voters, except insofar as it is linked to immigration levels.
The EU per se, and the issue of sovereignty, does, of course, matter greatly to many Conservative as well as UKIP activists.
It would be a surprise if UKIP were to win more than a handful of seats in the general election, but they could still have an influence on the outcome, taking more votes from the Conservatives than from Labour.
The big question is just how great the difference between Conservative and Labour defections to them will be - perhaps very great, and to Labour's electoral advantage, if it can convince its core voters that the party has not lost touch with them and its roots.
The Green Party may or may not improve on the single House of Commons seat it holds at present, but it has the potential to affect the outcome in a number of constituencies.
It may be the mass media's leader-fixation that prevents the Greens getting the attention their level of support, especially among young voters, merits.
How many people stopped on the street could name the present leader of the Green party? No, not Caroline Lucas - Natalie Bennett.
The Greens' more collective leadership may have hindered them from getting their fair share of media coverage, but it has not prevented their overtaking the Liberal Democrats in popular support - 7% nationally as against the Lib Dems' 6% in the last You Gov poll before Christmas.
The Greens are sure to eat into the Lib Dem vote in the May election, and they pose a danger to Labour in some marginal seats.
Of all the parties other than Conservative and Labour, including the far from inconsequential Northern Ireland parties, it is, however, the Scottish National Party which - uniquely for a UK general election - may hold the key to the result.
They are, by any objective measure, a more serious party than UKIP and are likely to win far more seats.
Unlike UKIP, to which they are sometimes, misleadingly, compared, they have demonstrated that they can govern and can do so effectively.
Having held office as a minority government in Scotland from 2007 to 2011, they won an outright majority in 2011, notwithstanding a highly proportional electoral system designed to ensure that no party (especially not the SNP) would ever have a monopoly of government posts.
Until now Scots have voted differently for the Westminster Parliament than for that in Edinburgh.
Not only did Labour win 41 out of 59 Scottish seats in the 2010 general election, it also increased its share of the vote in Scotland by 2.5% while dropping by 6.5% in the UK as a whole.
Labour's chances of an overall majority in the House of Commons after 7 May must depend on them holding on to most of those seats. At the moment this looks unlikely.
A December 2014 ICM poll of voting intentions for the general election put the SNP 17 points ahead of Labour, with the other parties trailing far behind.
Unless Labour can regain its former credibility north of the border, a majority of Scottish members of the House of Commons are going to be nationalists.
The rise of the SNP over the past two decades has often been attributed to the exceptional ability of its leader, Alex Salmond.
There is no denying his political talent, but yet again this is a case of a particular leader being used to explain too much.
Following the seamless transition to the leadership of Nicola Sturgeon - also, indeed, a formidable politician - support for the party has simply continued to grow.
The SNP have said that in no circumstances will they prop up a Conservative government, but - at a price - they might uphold a Labour administration.
That has very far-reaching implications.
If it led to the predictable English backlash, this would be grist for the mill of the separatist party, but bad news in the longer term for Labour - and for the continuing existence of the British state within its present boundaries.
- Archie Brown is emeritus professor of politics, University of Oxford, and the author, most recently, of The Myth of the Strong Leader: Political Leadership in the Modern Age