Now that the shock of Lise St-Denis’ floor crossing has worn off, it’s worth looking ahead to whether Jean Chretien’s old riding will stay Liberal red in the next election. Eric Grenier at 308 likes her chances:

If Lise St-Denis, the MP for Saint-Maurice–Champlain who defected to the Liberals from the NDP last week, decides to run for re-election under her new party’s banner, history suggests that, despite the wide margin between the two parties in the last election, she would have a good chance of winning the seat again.

[…]

Of those 34 floor-crossers who have stood for re-election, 22 were successful – including Richard John Cartwright. The success rate of floor crossers stands at 65 per cent.

[…]

More than half of floor crossers lose support when they run for re-election with their new party, with their average drop in vote share being eight points from one election to the next. However, the parties that welcome the new MPs almost always do better when the floor crosser runs for re-election. Fully 86 per cent of floor crossers have increased the vote share of their new parties in their riding, increasing their party’s support by an average of 11 points.

Ms. St-Denis received 39 per cent of the vote in Saint-Maurice–Champlain in May 2011, while the Liberals took 12 per cent. Applying these numbers to the average change in support when an MP has crossed the floor indicates she might expect to receive somewhere between 23 and 31 per cent of the vote in the next election, perhaps only enough to win a narrow victory.

There are a lot of ways to crunch the numbers, but before we try to draw conclusions based on 34 incredibly different data points from the past hundred plus years, we should probably look at the numbers that matter most – the votes in St. Maurice Champlain last election:

NDP 39%
BQ 29%
CPC 18%
Lib 12%

In terms of vote percentage, the riding ranked 33rd for the Liberals in Quebec (a sign of just how awful they did in La Belle Province last election). Maybe their electoral chances are better than that though – the 27 points the Liberals lost by makes it their 24th “best” riding in Quebec.

That’s not to say the Liberals can’t ride a 20 to 30 point red wave and win the seat next election, but it’s preposterous to argue St-Denis herself will have anything to do with that. Well known and well liked incumbents tend to be worth about 5 percentage points at the ballot box, and I wouldn’t use either of those terms to describe St-Denis. After all, she herself admits to being a no-name MP who was elected on Jack Layton’s coat tails.

The problem with looking at the historical numbers is that there’s little historical precedent for what St-Denis did. When Belinda Stronach moved from the Conservatives to the Liberals in 2005, she was jumping between parties which had been within 700 votes of each other in the previous election. In comparison, St-Denis just “gave up” 13,000 votes with her move. The more apt analogy might be kamikaze PC MP Jack Horner who won the bluest of blue ridings in Canada, Crowfoot, by 61 percentage points over the Liberals in 1974. He crossed the floor to take a position in Pierre Trudeau’s Cabinet and in the next election…lost by 59 percentage points. Yes, Jean Chretien’s old riding isn’t as hostile to the grits as rural Alberta during the Trudeau years, but Horner was a PC leadership candidate and had represented the riding for 20 years. All that was worth a few votes at the ballot box.

Let’s have some fun with numbers and create a single variable to measure just how much of an impact floor crossers have. I’ll take a page from baseball stat geeks and call it VORC – Value Over Replacement Candidate. Taking the Belinda Stronach example, the Liberal vote in the ridings around Newmarket Aurora fell 4 points between the 2004 and 2006 elections. So the fact that the Liberal vote actually went up by 5 points in Newmarket Aurora with her name on the ballot means she may have been worth 9 points. Of course, there are a gazillion other factors to consider, but it’s the best quick and dirty estimator of a candidate’s value we have.

So Belinda may have been worth 9 points at the ballot box, but using the same math, Garth Turner was worth a big “0” points to the Liberals in Halton in 2008, while Wajid Khan actually cost the Tories 3 points in Mississauga-Streetsville (KHAAAAAAAN!). So there’s no hard and fast rule.

Which, after that mathematical detour, is the point I’m trying to make with this post. When it comes to unique political events like by elections or floor crossings, it’s futile to look at past trends and averages to predict the future. Regardless of what may have happened to Jack Horner or Belinda Stronach or 1869 floor crosser Richard John Cartwright, Lise St-Denis’ situation is wholly unique – there aren’t many cases of first time MPs jumping to the party which finished fourth in their home riding.

So without precedent, all we can go on is political instinct and my political instincts tell me St-Denis’ value in the next election (should she even run, which seems unlikely) is negligible. That’s not to say her jump to the Liberals won’t help the party create a more favourable narrative province-wide, but on the ground in St. Maurice it won’t make a lick of difference come the 2015 election.