Is the expenses scandal driving turnout down?

Tonight's Populus poll also apparently shows that only 45% rate their chances of voting in an immediate general election at 10/10. This compares to 51% in Populus's last poll, and 57% in the one before the expenses row began.

Populus don't normally include the details of the likelihood to vote question in their published tables, so we can't really put it in a wider context. ICM however do, and we can see a similar pattern there. The graph below shows likelihood to vote in ICM's polls since early 2006 when they began including it in their standard published tables.

The blue line is the proportion of people who rate their chances of voting at 10/10, the orange line people's average answer. On both you can see the sharp drop in ICM's last poll, which equalled the lowest proportion of certain to votes (47%) since 2006, and was the lowest average likelihood to vote (7.25).