However, Clegg's tactic of ducking the issue as reflecting a fantastical "photo finish" scenario doesn't fit the electoral map. With a Tory lead anywhere between 0 and 4 points, Labour having more seats is very much the most likely outcome. With a Labour lead, or if the Tories were more than 6 points ahead, the issue is unlikely to arise. (If it really were a neck and neck photo finish in votes, Labour might well have an overall majority of its own). But the Clegg formula maintains wriggle room too: he concedes simply that a party with more seats and votes but no majority "has the first moral right to seek to govern on its own or reach out to other parties". There is still plenty to negotiate over, even if it turns out to be only over the terms of LibDem abstention on "supply and confidence".
Still, former LibDem Andrew Adonis isn't impressed by the Clegg formula, though I can not see so much point in challenging it now.
As the historically minded Adonia was friend and biographer of Roy Jenkins, he might also have noted that the Clegg formula did not apply to what LibDems have good reason to argue were the most important elections of the 20th century.
There were two elections in 1910, leading to what David McKie calls the "hung-est of Parliaments" (though 1923's three-way hung Parliament has a better claim there).
In January 1910, the Tories and Unionists won 2.9 million votes (46.8%) and the Liberals 2.7 million (43.9%), with Labour's 70 candidates winning 7% of the national vote. Yet the Liberals just squeaked ahead by 274 seats to 272.
By December, the Tories won 2.27 million votes (46.6%) and the Liberals 2.16 million (43.2%) and this time the seats for the two largest parties were tied on 272 each, with 42 Labour MPs and over 80 Irish Nationalists.
And more was at stake in 1910 than in any 20th century British election.
The major parties fundamentally disagreed over Lloyd George's great People's Budget, and the full veto of the fully hereditary Lords (used without restraint and for entirely partisan reasons by its Tory majority). The democratic issue was whether Britain could become a modern democratic country, or would seek to maintain a "balanced" polity with an aristocratic class veto to avoid Lord Salisbury's nightmare scenario where, absurdly, "two day labourers shall outvote Baron Rothschild'.
As I wrote in my new year essay on the lessons of 1910, "the most underrated year in British political history", the "votes or seats" question did not arise. There was no doubt what the Labour and Irish Nationalist MPs would do on the defining issues of the budget and the Lords:
1910 was a triumph for Liberalism. But it was a triumph made possible by a progressive Lib-Lab alliance. The Tories had a plurality of the popular vote in both 1910 elections, and were tied for seats. The Labour party did not ask which party had most votes or seats, but chose its alliances in that hung Parliament on values and principles. That alliance achieved a historic democratic breakthrough on which the later legislative achievements of Labour governments also depended.
Hence my theory that just about all of the great progressive advances in British politics arose from various forms of Lab-Lib co-operation.
The Labour party entered Parliament in 1906 through the (secret) Lib-Lab pact of 1903, and helped to achieve the Liberal democratic breakthrough of 1910-11.
The great welfare settlement of the Attlee governments after 1945 enshrined the ideas of Beveridge and Keynes, and so set the contours of British politics for three decades.
The social liberalisation of the 1960s was led by the great liberal icon Roy Jenkins as a Labour Home Secretary, while the abortion bill was introduced by David Steel and carried with much Labour backbench support.
The post-1997 constitutional reforms, the most significant changes to British polity since 1911, were achieved by the educative force of the liberal pressure group Charter 88 on Labour opinion, and by Lab-Lib cooperation in opposition and briefly in Cabinet committee during Labour's first term.
At other times - in dealing with the unemployment of 1920s and '30s, or facing the Thatcher challenge of the '80s, - these progressive forces have been scattered and divided, resulting in Tory dominance.
By contrast, what can be said for Liberal-Tory cooperation? There has only been one period of sustained Liberal-Tory alliance at Westminster, with Lloyd George becoming premier in 1916 and leading a Tory dominated government from 1918 to 1922.
This too had an enormous historic impact: it split the Liberals irrevocably in the coupon election of 1918, ending their status as a national party of government, saw the government fail to honour its progressive social promise of "homes fit for heroes", and was in any event ended by the right-wing Tory backbench revolt at the Carlton Club, celebrated in Westminster to this day in the name of the 1922 committee.