SO now the Lib Dems are faced with an offer Labour believes they cannot refuse — a rainbow coalition made up of every colour except Brown.

It came thanks to an act of ritual self-sacrifice by Gordon Brown, tendering his resignation as Labour leader, promising to stay in No 10 as a mere caretaker. In so doing, the outgoing prime minister hoped he would remove what had proved to be an insurmountable obstacle standing in the way of his last great ambition, a project that, should he achieve it, would surely count as his most unexpected legacy the creation of a progressive, reforming coalition made up of Labour and the Liberal Democrats.

Until Monday evening, when Brown stepped into the daylight outside No 10, the Lib Dems — buffeted by offer and counter-offer from the Conservatives and Labour — faced what they regarded as an uneven contest.

The Tories had the edge on the two criteria that mattered, promising both stability and legitimacy. Any Con-Lib coalition would not only have solid numbers in the Commons but could claim a moral right to govern, led by a Tory party which had passed Clegg's “most seats and votes” test.

Before Brown's statement, Lib Dems believed Labour could not match the Tories on either count. Yes, Labour might feel like natural bedfellows to most Lib Dem supporters, but any progressive coalition they might cobble together would be perilously frail — reliant on assorted Irish and Welsh nationalists and a sole Green MP to march in lockstep with every last member of the Labour and Lib Dem parliamentary parties. What's more, feared Clegg, any coalition with Brown at its head would lack legitimacy, led by a prime minister rejected by the voters.

Now the Lib Dem leader has been handed a partial answer to the second of those problems. Senior Lib Dems say there's a kind of “psychological legitimacy” in a centre-left alliance that will, in a few months, have a new face at the top. Labour's new leader won't have come through a general election — which will surely bring howls of outrage — but he or she will, say the Lib Dems, at least have been elected by his own party.

The trouble for Clegg is that he has no idea, and no influence over, who that person will be. Labour are inviting Clegg on to a dance floor shrouded in darkness, allowing him to see the face of his partner only once the lights come on.

— The Guardian, London