Start with this: the Conservatives will probably gain more votes than the polls are currently suggesting. There's a natural 'swingback' towards the governing party in the days leading up to a General Election, as discontent with any governors is replaced by a focused choice between two alternative administrations (and Prime Ministers). How big will this be? Well, there are a number of ways of looking at this, from statistical models that suggest that the Conservatives will have a few more MPs than Labour, to more straightforward looks at the polls since the 1980s - which also suggest that the Conservatives may well come back to be the biggest group in the Commons, but probably won't gain an overall majority.
We can go further back into Britain's post-1945 past, though - not to strictly comparable data, because polling changed out of all recognition after its debacle of a result in the 1992 General Election. But to gain some impression of how governments have done in hauling their way back into public favour in the year before General Elections. And the answer? Well, it's a mixed bag.
The Labour Government of Clement Attlee led by three points in the Gallup Poll of October 1950; they went on narrowly to lose the next election a year later (though they won more votes than Winston Churchill's Conservatives); the Conservatives were two points behind a year before the 1955 election, which they went on to win by over three points; in October 1958 Harold Macmillan enjoyed an opinion poll lead of four points, going on to win the General Election a year later by six points.
That's when it gets more hopeful for the Conservatives. They were twelve points behind Labour in October 1963, but they lost by only a tiny margin in 1964; Harold Wilson's government was a massive sixteen points behind in 1969, but lost out in 1970 by only three points. However, if we scroll forwards to early 1978, Labour leader Jim Callaghan enjoyed a rough partity with Mrs Thatcher's Conservatives, only to get hammered in spring 1979 after the Winter of Discontent had discredited Labour's links with the trade unions.
So electoral and polling history does point towards the Conservatives doing much better in May 2015 than the polls suggest right now. But an overall majority? That's their real, and perhaps unrealisable, target - without which David Cameron (above) might find himself faced with a party that regards him as a two-time loser.