The Greens are celebrating a historic breakthrough in Prince Edward Island today after Tuesday's provincial election. They're also experiencing a pang of disappointment.

It's a familiar feeling for Greens across the country.

The party has long been unable to deliver at the ballot box the support that has showed up for it in public opinion polls. While local factors contributed to the P.E.I. Greens failing to live up to the lofty expectations set by the polls going into the vote, their under-performance on the Island is par for the course for Greens in election after election, from the Pacific to the Gulf of St. Lawrence.

With a week to go before Tuesday's election, the Greens were leading in the polls. Three different surveys gave the party between 35 and 40 per cent support, putting them well ahead of the Progressive Conservatives, who scored between 29 and 32 per cent.

A poll conducted over the last weekend by Forum Research showed the Tories inching ahead by a margin of 35 to 34 per cent, but the survey still showed the Greens in a strong position.

Instead, the final results in 26 of 27 districts (the vote in one Charlottetown district was postponed after the death of Josh Underhay, the Green candidate) showed the PCs with 36.5 per cent of ballots cast and 12 seats. The Greens finished with 30.6 per cent of the vote — 5.6 percentage points worse than their average in the four polls published during the campaign.

P.E.I. Green Party leader Peter Bevan-Baker discusses his party being the first Green Party in Canada to be elected as official opposition. 6:56

A historic breakthrough for the Greens

The Greens have a lot to be proud of in Tuesday's results. Before the P.E.I. election, the party had never taken more than 16.8 per cent of the vote in any provincial or federal contest; that record was set in the 2017 British Columbia provincial election. The Greens nearly doubled that result in P.E.I.

The eight seats Peter Bevan-Baker's P.E.I. Greens won matches the combined total of Green legislators in B.C., Ontario and New Brunswick.

Even when viewed solely within the P.E.I. context, the results were remarkable. In all the history of its general elections before Tuesday, P.E.I. had elected only two MLAs from parties other than the Liberals or the PCs. It elected eight of them on Tuesday. The province had never had a minority government. It just elected its first.

Nevertheless, the results offer another example of the Greens coming up short — a problem that the party has yet to solve.

Greens nearly always under-perform the polls

Since 2004, when the Green Party of Canada first ran a full slate of candidates, the party and its provincial cousins have put up candidates in at least a majority of ridings or districts in 34 elections. Of the 30 elections in which polling was conducted in the last week of the campaign (and where the data are still available), the Greens under-performed their polls in 27 of them — 90 per cent of the time — by an average of 1.5 points.

Even in the three instances when the Greens beat their polls, they over-achieved by less than a percentage point. Adjusting for the elections in which the Greens did not run a full slate still shows the party performing worse than its polling more than two-thirds of the time.

By under-shooting the polls by 5.6 points in P.E.I., the party hit a new low. Still, as a share of its overall support, its performance Tuesday night was about even with how Greens have done across the country over the last 15 years. On average, what the party gets at the ballot box works out to about four-fifths of the support attributed to it in pre-vote polling.

Tuesday night's results certainly can be chalked up in part to the specific dynamics of this election. The Greens had far fewer resources in P.E.I. than either the Liberals or the PCs, and ran less experienced get-out-the-vote operations. The PCs and Liberals had many more incumbents — a key factor in a province where the local candidate carries much more weight in voters' decisions than almost anywhere else in Canada.

Accordingly, both the PCs and Liberals out-performed their polls. That was enough to propel PC Leader Dennis King to the premier's office. It wasn't enough to protect incumbent Liberal premier Wade MacLauchlan from losing his own seat.

But there are still lessons to be learned here for Greens elsewhere.

Amanda Alvaro, Tim Powers, Chris Hall and Marcella Munro discuss what the P.E.I. election means for the rest of Canada. 9:33

Lessons for off-island Greens

Unless a party is riding the cusp of a wave — as the New Democrats did in Quebec during the 2011 federal election — organization and resources can make a big difference. And unlike long-established political brands like the PCs or Liberals, the Green Party can't count on voters to pick their local candidate solely because of the party's brand or the popularity of the leader (Bevan-Baker out-polled King and MacLauchlan by wide margins). So Green parties need particularly strong local candidates.

Tuesday's election also serves as a warning to Greens to tamp down expectations. The party is polling at 8.5 per cent nationwide, according to the CBC Poll Tracker. That would give the federal Greens their best performance ever — if they can hang on to that vote until October. The Poll Tracker estimates that the Greens are in contention in as many as seven ridings across the country.

The polls are not the only signs that the Greens are trending upwards. The P.E.I. election is just the latest in a series of breakthroughs the party has managed in the last few years in provincial elections from B.C. to New Brunswick.

But until the Greens can figure out how to turn promising polls into actual votes, the party could end up missing more opportunities.