Conservative MP Ted Opitz and former Liberal MP Borys Wrzesnewskyj are facing off in the Supreme Court of Canada today over the House of Commons seat in Toronto's Etobicoke Centre.

Opitz won the riding in the 2011 federal election, but Wrzesnewskyj argued there were clerical errors by Elections Canada that meant some of the ballots shouldn't have been counted. Wrzesnewskyj won a victory in Ontario Superior Court in May, leading to Opitz's appeal to the country's top court.

The Supreme Court is holding a special hearing, outside of its normal schedule, to speed up the legal process. This is the last arena for Opitz to argue to keep his job and for Wrzesnewskyj to make his case for a new election in the riding.

Here are four things you should know about the case:

1. Tuesday's legal arguments

Lawyers for both Opitz and Wrznewskyj will make their cases in the Supreme Court hearing. The top court justices will also hear from interveners who want to make points about the case, including Elections Canada.

Supreme Court cases normally take months, even years, to render a decision. In this case, the court will speed up its normal process, but that could mean the reasons for the ruling won't be released until months later.

2. What the case is about

Wrzesnewskyj, who lost the seat to Opitz on May 2, 2011, says Elections Canada made a number of clerical errors that meant they couldn't prove all the ballots cast came from people living in the riding. If voters arrive at a polling station without a voter identification card, they can sign a registration certificate to swear they live in the area. But some of those registration certificates weren't signed or just didn't exist. In other cases, the workers employed for the writ period by Elections Canada vouched for more than one person, which isn't allowed under the agency's rules.

Ontario Superior Court Judge Thomas Lederer found that 79 ballots shouldn't have been counted because of those errors, and those ballots outnumbered the 26-vote margin by which Opitz won. Because of the number of ballots Lederer threw out, the election result was declared null and void. Opitz is challenging Lederer's decision, arguing those 79 ballots are valid even though mistakes were made at the polls.

Elections Canada has since found that 44 of the voters whose ballots were disqualified were on the National Register of Electors, meaning they are Canadian citizens and qualified to vote. But the documents tabled in advance of the court case have the voters' addresses blacked out, so it's not clear whether they live in the right area to have voted where they did. A code next to the redacted addresses shows almost half of them didn't live in the riding the month before the election.

While Opitz is appealing the lower court's decision, Wrzesnewskyj is appealing some of the findings — that people can vote in the wrong polling division as long as they vote in the right riding, and the decision to allow a set of ballots that Wrzesnewskyj says are from people who voted twice.

3. Ruling could lead to byelection

If the court finds in favour of Opitz and overturns the lower court’s ruling, that would keep the result of the 2011 ballot in Etobicoke Centre. Opitz would remain the MP.

If the Supreme Court agrees with the lower court, Opitz will no longer be the MP for Etobicoke Centre. If the court upholds the Ontario court’s decision, voters will face a byelection, one of three coming up to fill the seats left by departing Conservatives. Calgary Centre MP Lee Richardson and Durham, Ont., MP Bev Oda are both retiring from the House of Commons.

4. A byelection could be 6 months away

If the Supreme Court upholds the Ontario Superior Court decision, the seat will be declared vacant. Prime Minister Stephen Harper has six months from then to announce a date for the byelection. Opitz and Wrzesnewskyj both intend to run to reclaim it.

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