A Lib Dem West Country MP told me at the start of the year that he thought his party would keep his seat if the Tories were broadly ahead in the national polls on polling day but lose it if they were level or behind. His thinking was that if it looked like Cameron was going to continue as Prime Minister his constituents would both feel it was safe to vote for a local champion and would want some protection against the Tories cutting public services too far. But if the Tories were behind, he feared that these swing voters would feel that they had to vote Tory to try and stop Labour.
This is what makes the current polls particularly concerning for the Lib Dems. If they don’t move, then they could face a bigger squeeze than expected in their Tory facing seats.