The Conservatives have put up poor results in byelections since winning a majority government in 2011, and will again be put to the test in two contests in Ontario and Alberta on Monday.

Will they be able to hold on to their two seats?

The byelections in the Alberta riding of Yellowhead and the Ontario riding of Whitby–Oshawa follow the resignation of former Conservative MP Rob Merrifield and the death of former finance minister Jim Flaherty.

Both seats were solid Conservative victories in 2011. Flaherty captured 58 per cent of the vote in Whitby–Oshawa, outpacing the NDP by 36 points. The Liberals finished third in the riding with 14 per cent, while the Greens took five per cent. In Yellowhead, Merrifield took 77 per cent of the vote, against 13 per cent for the NDP, five per cent for the Greens and just three per cent for the Liberal candidate.

In light of these margins, both seats would be considered safe Conservative holds.

However, the Tories have taken a hit in the polls in both Alberta and Ontario.

In their western bastion, the Conservatives are currently averaging about 53 per cent support, a drop of 14 points since the last election. In Ontario, the Conservatives have dropped 11 points to about 33 per cent. New Democrat support has also declined, by three points to 14 per cent in Alberta and seven points to 19 per cent in Ontario. This would seemingly put them out of contention for the byelections.

The Liberals, on the other hand, are in a far stronger position than they were in three years ago.

In Alberta, the party is averaging about 21 per cent support, up 12 points since 2011, while the Liberals are leading in Ontario with 40 per cent, an increase of 15 points.

While the margin may be too wide to overcome in Yellowhead, this certainly does make the Liberals capable of challenging the Conservative hold in Whitby–Oshawa.

Conservatives lose, Liberals gain in byelections

Even without the polls, there would be reason to expect the Conservative margins of victory to be reduced in these two byelections. The governing party has been on the receiving end of some significant setbacks in byelections since the 2011 vote.

In the 13 byelections held since the last federal election, the Conservatives have suffered an average loss of 11.5 points, or roughly a third of the support they had in the previous general election. In every contest, their vote share has dropped.

The Official Opposition New Democrats have not been the beneficiary, having lost an average of 6.3 points per byelection (or about a quarter of their vote share). Instead, the Liberals have been making extraordinary gains.

Before Justin Trudeau took over the party, the Liberals averaged a gain of 6.2 points per byelection in 2011 and 2012. But since he became leader, the Liberals have experienced an average gain of 18.6 points, roughly tripling their vote share.

Moreover, under Trudeau, the Liberals have not dropped in vote share in any byelection and have never gained less than seven points. This has allowed them to win byelections in which they were not the incumbent party in Labrador and Trinity–Spadina, as well as turning seemingly insurmountable margins into close races in unexpected places like Fort McMurray–Athabasca and Brandon–Souris.

These sort of swings, significant as they are, may not be enough to flip either Yellowhead or Whitby–Oshawa. The swing of 30.5 points between the Conservatives and Liberals in byelections since Trudeau's arrival on the scene would not overcome the margins that existed between the two parties in both ridings in 2011 (44.3 points in Whitby–Oshawa, 74.1 points in Yellowhead). However, if the Liberals do manage to triple their vote share again, that would put Whitby–Oshawa in play.

Are byelections always rough for governments?

A standard line provided by the governing party in any jurisdiction is that byelections are routinely bad for a government, and that losses are to be expected. While that has been the case for the current Conservative term, it is a recent exception.

While Stephen Harper presided over two minority governments between 2006 and 2011, his party actually increased its share of the vote in byelections. Between the 2006 and 2008 federal elections, the Conservatives gained an average of 3.8 points per byelection, seeing an uptick in support in six of nine contests. Between the 2008 and 2011 votes, the Conservatives increased their vote share by an average of 1.3 points (excluding the 2009 byelection in Cumberland–Colchester–Musquodoboit Valley, in which a popular independent had resigned, the Conservatives' average increase is even greater if this byelection is included).

CP's Baloney Meter Do byelections punish the governing party?

What changed after 2011? The shift in byelection fortunes might be chalked up to the transition from a minority to a majority government, except that the last majority government (that of the Liberals between 2000 and 2004) experienced an average gain of 5.2 points per byelection.

So the Conservatives' poor performances in byelections over the last three years cannot be considered routine, the normal punishment voters deal out to parties in power. Rather, their performances have matched the Conservatives' slip in national voting intentions, backing up those unverifiable, ephemeral polling numbers with real voting results.

The Conservatives may be safe in Yellowhead and Whitby–Oshawa Monday, but if they repeat these performances in the next general election, the Conservatives will quickly find themselves on the opposition benches.

ThreeHundredEight.com's polling averages include all publicly release opinion polls, weighted by age, sample size and the track record of the polling firm. Methodology, sample size and margin of error if one can be stated vary from survey to survey and have not been individually verified. For full methodology, see here.