For more than a decade now, whenever Ipsos Mori conducts a poll of voting intentions, we published two sets of figures: which party everybody who expresses an opinion says they would support if there were a general election tomorrow, and which parties those people who are certain they would vote would support.
The proportion of each party's supporters who are certain to vote goes up and down from one poll to another and tends to rise for both parties as an election approaches, but, despite the fluctuation, up to 2010 the Conservative advantage never entirely disappeared.
However, as the chart shows, things have changed dramatically since the general election. Labour supporters have tended to be much more sure they would vote than was the case when the party was still in office, and for the last year-and-a-half or so the Conservatives have also been noticeably less certain of voting than they used to be. As a result, the gap between the two lines has narrowed substantially, and there have even been a few of our monthly polls when those who said they would vote Labour have been more likely than Conservatives to say they are certain they would vote.
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