The dilemma for Lynton Crosby and the Tories: Labour can win with a much lower vote share than the Tories- A problem that not even Lynton Crosby can resolve

The reality is that on this projection Labour would gain a small majority, firstly because the Sinn Fein MPs in Ireland do not take their seats in the UK Parliament

The Tories have a major dilemma which the Labour Party do not, which puts them at an immediate disadvantage. It is a dilemma that not even Lynton Crosby can do anything about. The problem is that Labour is so much more efficient at getting voters out to vote for them for a variety of reasons.

One of the major reasons is the constituency boundaries. Labour seats on average contain 2000-3000 less people than Tory constituencies. This is because the last time constituency boundaries were significantly changed was in 1997, just before the Tony Blair landslide. The constituency boundaries have remained relatively unchanged since then. The Tories last year attempted to redraw constituency boundaries and decrease the number of MPs to 600 from 650, which would have led to a net increase of 12 Tory MPs and a larger decline in the Number of Labour and Lib Dem MPs. Unsurprisingly, these proposed boundaries were slammed by Nick Clegg as being "Unconstitutional" and the Lib Dems voted against the proposed changes in retaliation for Tory support for the NO2AV campaign during the referendum campaign in 2011, which put those ideas to bed. Thus, for the next general election at least, the Conservatives are at an immediate disadvantage.

The big driver of the so-called "Bias" toward Labour in FPTP is that LAB voters are generally more reluctant to turn out where the outcome is not in doubt, particularly in Conservative and Labour heartlands. Where it does matter, in the marginals as we’ve seen election after election after election, the Labour machine gets its vote out and is particularly skilled at securing anti-CON tactical votes from potential Liberal Democrat voters (And next time possibly from UKIP voters) This explains the disproportionate fall in Tory support in key marginals, which puts far more seats at risk for the Tories than it would be on a uniform national swing. What do the Tories do? The aborted boundary plan would have added about 12 seats to the party total which would have helped some way, but only so far.

Differential LAB turnout and tactical voting are what does for the Tories and the signs are that these could happen again.On my calculations, if the ashcroft polls are seen as the norm, Labour could win a majority despite being 2% behind in the vote share, and will remain the largest party until the difference becomes at least 5 clear percentage points. The percentage needed for even a tiny outright Tory win would be 10-12 points ahead of Labour- I.e to even remain the largest party next time, the Tories actually need to improve on their 2010 performance.

Does this make a Tory majority impossible?
No.

But it does make it extremely unlikely, which in political terms means pretty much the same thing.
Whether or not the Tories come out as the largest party is another matter entirely.