The editorial will disappoint Guardian-reading Labour supporters, though it was always likely before the LibDems advanced in the polls since The Guardian had vociferously argued for Labour to drop Gordon Brown as leader last year, and endorsed the LibDems at the European elections.
The paper's line also reflects a readership already evenly split between Labour and LibDem supporters - dividing 48% Labour, 34% LibDem and 7% Tory ahead of the 2005 election according to MORI, suggesting the LibDems would need a 7% swing to secure a plurality among Guardian readers next week.
The Guardian editorial says that it remains a "newspaper that is proudly rooted in the liberal as well as the labour tradition", setting out that it is in 2010 now a Lib-Lab paper, rather than a Lab-Lib one.
If the endorsement is new, The Guardian has supported a greater LibDem Parliamentary presence in every election since the 1988 merger, backing the party or its predecessors in some form in nine out of ten general elections since 1974 (including this one), while supporting Labour in only seven of the same elections. British Political Facts suggests it backed Labour alone in 1987, but The Guardian did not endorse Labour in supporting a Liberal vote for a hung Parliament in February 1974, and the Liberals in the second election, nor in 1983 in supporting the Alliance (wanting it to govern in coalition with the Conservatives). I am relying on British Political Facts - and can't pretend to have all of the clips!
However, the 2010 editorial does significantly shifts the balance of its 2005 editorial argued that "We want to see Labour re-elected to government and we want to see more Liberal Democrats returned to parliament, at whichever other party's expense".
In 2005, the paper also repeated its argument of 1997 that the paper recognised
"without embarrassment or apology that we, the Liberal Democrats and the Labour party are companions on the journey down the same broad river of progressive politics" adding that "In 2005, though we have all been tumbled about by some rapids, that remains true. The modern progressive consensus of which Gordon Brown speaks will be the work not of one party alone, but of two".
Though Labour supporters might challenge the conclusion, tomorrow's Guardian editorial makes a well argued and nuanced case about each of the three major parties.
It has an effective, engaged but critical account of David Cameron's political project, noting several ways in which Cameron has sought to bring his party into the poltiical centre, but regarding his policy agenda as thin and contradictory, criticising his neo-liberal response to the financial crisis, and worrying that the Cameronisation of the Conservative party"seems more palace coup than cultural revolution"
His difficulty is not that he is the "same old Tory". He isn't. The problem is that his revolution has not translated adequately into detailed policies, and remains highly contradictory. He embraces liberal Britain yet protests that Britain is broken because of liberal values. He is eloquent about the overmighty state but proposes to rip up the Human Rights Act which is the surest weapon against it. He talks about a Britain that will play a constructive role in Europe while aligning the Tories in the European parliament with some of the continent's wackier xenophobes. Behind the party leader's own engagement with green issues there stands a significant section of his party that still regards global warming as a liberal conspiracy.
The Tories have zigzagged through the financial crisis to an alarming degree, austerity here, spending pledges there. At times they have argued, against all reason, that Britain's economic malaise is down to overblown government, as opposed to the ravages of the market.
The Guardian also invokes Monty Python's People's Front of Judea to note how easily Labour's achievements in power are overlooked:
The salvation of the health service, major renovation of schools, the minimum wage, civil partnerships and the extension of protection for minority groups are heroic, not small achievements.
It is particularly critical of Labour on civil liberties and foreign policy, but does credit Labour with having a stronger record on poverty than its progressive rival
The Liberal Democrats are a very large party now, with support across the spectrum. But they remain in some respects a party of the middle and lower middle classes. Labour's record on poverty remains unmatched, and its link to the poor remains umbilical.
(This praise of an "umbilical link" sits oddly with a critical reference to both Tory and Labour links to "reactionary and sectional class interests" slightly odd: that sounds much more like a generic attack on trade unions as a whole than a challenge to particular organisations or individuals within them).
But for The Guardian, the primary issue is electoral and political reform, along with broader civil liberties and Europe as issues on which the newspaper's priorities are closest to the LibDems.
The paper is hopeful that"this will be Britain's last general election under a first-past-the-post electoral system which is wholly unsuited to the political needs of a grown-up 21st-century democracy" and believes the election could result in a LibDem and Labour partnership which could achieve that.
However, its 2005 editorial quoted the paper's best known editor CP Scott.
"It is quite possible that while Liberalism and Labour are snapping and snarling at each other the Conservative dog may run away with the bone. That would be lamentable."
If such an outcome would have been lamentable in the early 20th century, it would be just as lamentable in the early 21st.