David Miliband's declaration that "In 2015 Labour can win the general election and Ed can be in Downing Street", the warmest endorsement he has delivered of his brother's leadership, is a reminder of how much the political mood has changed in recent months. Labour's sustained poll lead - the latest YouGov poll puts them 10 points ahead of the Conservatives - has forced the Tories to confront what they previously thought unthinkable: that Ed Miliband could become Prime Minister.

When Miliband became Labour leader in September 2010, "unelectable" was the most common epithet applied to him by the right. This was partly due to history. As David noted in his speech last night - Ministers and Politics in a Time of Crisis - Britain's recent experience of long-lived governments (18 years for the Tories, 13 years for Labour) led to a casual assumption among the media that the Tories were destined for a second term:

Younger listeners may not know this, but governments can actually lose elections before they win three in a row. In the 1970s there were four prime ministers and five governments in nine years. For me and my party, this is great news. In 2015 Labour can win the general election and Ed can be in Downing Street.

Miliband is right. We could be heading for a period of 70s-style revolving door government.

Poll leads, of course, can come and go. In February 1981, Michael Foot led Margaret Thatcher by 16 points. Yet aided by the "Falklands bounce", the Tories went on to win a majority of 144 seats in 1983. But the assumption that Labour was doomed to a lengthy spell in opposition was always at odds with psephological reality. At the 2010 election, the party may have recorded its second lowest share of the vote since 1918 (29 per cent) but, owing to the vagaries of the British electoral system, it still emerged with 258 MPs, far more than the Tories had in 1997 (165 MPs), 2001 (166 MPs) and 2005 (198 MPs). For Miliband, the road to a majority is far shorter than it was for Cameron.

One should add that while the Tories' planned boundary changes will reduce Labour's advantage, they will not eliminate it. Even after the changes are implemented, the Tories will need a lead of seven points on a uniform swing to win a majority (compared to one of 11 points at present), while Labour will need a lead of just four. The reason Labour retains its electoral advantage is that the electoral bias towards the party owes more to differential turnout (fewer people tend to vote in Labour constituencies) and regional factors (the Tory vote is poorly distributed) than it does to unequal constituencies (the coalition plans to fix constituency sizes at around 76,000 voters).

Another factor in Miliband's favour is Labour's status as the least toxic party. While just 58 per cent of the electorate would consider voting for the Tories, 70 per cent would be prepared to back Labour. Or, to put it another way, just 30 per cent would “never” choose Labour compared with 36 per cent for the Lib Dems and 42 per cent for the Tories. Thus, it is Labour that has the greatest potential to expand its support.

Finally, as Mehdi noted in a recent column on this subject, Cameron will need to defy recent history to achieve a majority. To win the election, the PM needs to significantly increase the Tories' vote share from the 36 per cent they polled at the last election. Yet "not since 1974 has an incumbent prime minister pushed up his party's share of the vote. It was beyond the ability of Margaret Thatcher (in 1983 and 1987) and Tony Blair (2001 and 2005)". Some Tories have already recognised that Cameron will struggle to win a majority (see Tim Montgomerie's recent pieces on the subject), it is time the rest of the party did the same.