I've been playing around with the idea of a swingometer for the upcoming provincial election. The efficacy of this is complicated by the high poll numbers for the NDP.
The concept has been used in the UK for half a century.
A simple explanation is this: they line up the ridings in order of strongest for major party A to weakest for major party A (and therefore strongest for major party B). They then compute the swing required for each seat to flip to the other party (half of the margin between the parties +1). They can therefore determine how many seats each party should win based on a particular swing in the popular vote.
Based on my seat model, the last election under the new boundaries would have resulted in 38 PCs and 11 Liberals. The PCs won the popular vote by 14.4 percentage points, and therefore a swing between the parties of 7.2 points in favour of the Liberals would result in a tie of the popular vote.
According to my calculations however, it would actually take a swing of 7.7 percentage points for the Liberals to win a majority government by taking Moncton South as the 25th and decisive seat.
A swing to the PCs of over 11.6 percentage points would be required for them to sweep every seat in the legislature. But a swing to the PCs of just over 1.7 percentage points would reduce the Liberals to 5 seats - showing just how close the last election was to a near wipe-out for the Liberals.
Conversely, the Liberals would need a swing of over 23.3 percentage points to take all of the seats.
The above is blind to the NDP. By design, the swingometer is a two-dimensional creature and can compare only two parties. However, the strength of the NDP changes the ordering slightly even if they're ignored for swingometer purposes.
This is what the swingometer looks like if we use the last CRA poll as a base. That poll had the Liberals at 43% and the PCs at 31% (the NDP was at 21%). That would indicate a result moving from PCs +14 at the 2010 election to PCs -12, a change of 26 percentage points and a swing of 13 percentage points. Using these numbers as a base, we would expect an election to yield 39 Liberals, 8 PCs and 2 NDPers.
As you can see, both of the NDP seats come at the expense of what would otherwise have been Liberal seats with this big of a swing. However, if we look at them in terms of road to majority government, the NDP is doing equal damage to both parties: taking an otherwise safe Liberal seat (Saint John Harbour) and an otherwise leans PC seat (Fredericton West-Hanwell).
In order to recover to a majority government from this poll number, the PCs need a swing of over 5.7 percentage points to win Southwest Miramichi-Bay du Vin and take 25 seats.