In recent weeks a new mood of pessimism has taken hold among Labour MPs and a new mood of optimism among the Tories. Labour's double digit poll lead has shrunk  to around eight points, with the party ahead by just five points in the last two YouGov surveys. Since governing parties almost always recover support in the months before the general election (as the opposition comes under greater scrutiny and as voters start to view the election as a choice between competing alternatives, rather than a referendum on the government), a lead of five is not enough for Labour to be confident of winning a majority in 2015 or even of becoming the largest single party.
Among MPs, a hung parliament, rather than a Labour majority, is now viewed as the most likely outcome of the election, with some even speculating that the Tories could go all the way and achieve the majority that eluded them in 2010. But for good reasons, the odds (literally ) are still on a Labour victory in 2015 (in this case, defined as becoming the largest party). It's true that oppositions rarely return to government at the first attempt of trying - the last time was in February 1974 - but Britain's new electoral landscape means Labour is well positioned to do so. Here are five of the reasons why.
1. Labour can win big on a small lead
The key point in Labour's favour is that it requires a far smaller lead than the Tories to win a majority. Based on a Lib Dem share  of 15 per cent, Labour needs a lead of just 1 per cent to win an overall majority, while the Tories require one of 7 per cent. In 2005, Labour won a majority of 66 sets with a lead of three points but in 2010 the Tories fell 20 short with a lead of seven. This apparent bias has less to do with the unreformed constituency boundaries than it does with the fact that Labour's vote is far better distributed than the Tories' and that it benefits disproportionately from tactical voting.
It's important to remember that uniform swing calculations are an unreliable guide to election outcomes since they don't take into account factors such as the incumbency bonus and above-average swings in marginal seats. Had there been a uniform swing in 2010, the Conservatives would have won  14 fewer seats, Labour eight more and the Lib Dems five more. But even if, as seems likely, the Tories perform disproportionately well in their existing seats, they will still to struggle to establish the lead that they need over Labour to even remain the largest party.
2. UKIP will still be a force in 2015
While UKIP's support is almost certain to fall heavily before May 2015 (they are currently polling around 12 per cent in YouGov surveys), it's likely that it will poll above 5 per cent, a level of support that is large enough to have a significant influence on the outcome of the election. With UKIP drawing around 60 per cent of its support from 2010 Tory voters, it is the Conservatives who will lose most from the rise of the Farageists.
The split in the right-wing vote will make it easier for Labour to dislodge the Tories in the marginals it needs to win to become the largest party. At the last election, with a UKIP share of just 3 per cent, there were 20 constituencies in which the party's vote exceeded the Labour majority (one shouldn't make the error of assuming that all those who supported the party would have backed the Tories in its absence, but many would have done).
3. Most Lib Dem defectors are likely to remain loyal to Labour
The main reason why Labour has led the Tories in the polls for more than two years, despite suffering its second worst defeat since 1918 at the last election, is the mass defection of left-leaning Lib Dems to the party in protest at the coalition. Significantly, as Lord Ashcroft's recent poll  of the party's supporters noted, they are less likely to return to the Lib Dem fold than other voters. Ashcroft observed that "those who have moved to Labour are the most likely to say they are sure how they will vote (78%). This compares to just over a two thirds of those who say they would vote Conservative (69%), just under two thirds of those who say they would vote UKIP (62%) and less than half of those who would vote Green (42%)."
The decision of the Lib Dems not to replace Nick Clegg with a more left-wing figure such as Vince Cable or Tim Farron makes it more likely that these voters will remain loyal to Labour in 2015.
4. Labour's brand is strong even if Miliband's isn't (and that may not matter)
Of the three main parties, it is Labour that is the least toxic, with 46 per cent of voters saying that they would "consider"  voting for the party compared to 40 per cent for the Tories. While 35 per cent say that they would "definitely not" vote for Labour, 43 per cent say that they would "definitely not" vote for the Tories, placing a notable cap on Conservative support. Labour, by contrast, is fishing in a larger pool.
Faced with this disadvantage, the Tories console themselves with the thought that Ed Miliband's unpopularity will deny his party victory (the Labour leader is known as their "secret weapon"). Miliband's approval rating is currently -33 compared to Cameron's -18 and the Tory leader has consistently led  as "the best prime minister" (most recently by 35-20).
But while Cameron's greater popularity could save the day for the Tories, it is complacent of the party to assume as much. History shows that a well-liked (or, more accurately, less disliked) leader is no guarantee of electoral success. In the final poll before the 1979 election, Jim Callaghan enjoyed  a 19-point lead over Margaret Thatcher as "the best prime minister" but that didn't stop the Conservatives winning a majority of 44 seats. Similarly, in the 1970 election, Harold Wilson's personal lead  over Ted Heath (a 51 per cent approval rating compared to one of 28 per cent for Heath) didn't stop Labour going down to a decisive defeat.
In 2010, David Cameron's lead over Gordon Brown wasn't enough to deliver the Tories a majority. In 2015, his lead over Miliband may not be enough to deny Labour victory.
5. The Lib Dem incumbency bonus will hurt the Tories
More than any other party, the Lib Dems benefit from an incumbency bonus, with their MPs typically polling  between 5 and 15 per cent more than the party's other candidates. As the Eastleigh by-election demonstrated, in those seats where the party is well organised and where it can appeal for tactical votes from Labour supporters, it can still win. In 2015, this will largely be a problem for the Tories, who are in second place in 37 of the Lib Dems' 57 seats. The potential for the Tories to make large gains from the Lib Dems to compensate for the marginal seats they will lose to Labour is limited.