The tumultuous and lightning-quick contest to become the new leader of the Ontario Progressive Conservatives enters its final phase Friday as party members begin voting.

However, thousands of them are not yet able to cast their ballots because of delays in receiving essential paperwork.

The PC party's online voting system opened at 9 a.m. Friday, and remains open until the evening of March 8. Party members will choose between Tanya Granic Allen, Christine Elliott, Doug Ford and Caroline Mulroney.

The winner will be announced March 10, less than three months ahead of Ontario's election day on June 7.

Many members have yet to receive a key document that they require to vote, according to information provided to CBC News from the four campaigns.

PC members must have their eligibility verified online. The process involves uploading identification and submitting a code sent from the party by mail. It`s to ensure the result is not torqued by fraudulently purchased memberships.

Caroline Mulroney's spokersperson said there is concern whether all party members will receive their verfication to vote in time. (David Donnelly/CBC)

"We have thousands of voicemails and thousands of emails from people who are complaining that they haven't received their codes," said Ford campaign press secretary Lyndsey Vanstone. "We can't keep up."

The Mulroney campaign surveyed supporters on Thursday night and an official says the majority of those who responded indicated their registration package had not arrived.

"We are very concerned about the ability of all members to vote in this election," said Mulroney spokesperson Melissa Lantsman in an email to CBC News. "Right now we do not have firm numbers, but anecdotally we know verification levels are low."

Verification codes delayed

Officials in the Elliott campaign said many of her supporters have not received their codes and are getting frustrated waiting.

All the campaigns are offering sessions to help people go through the verification process. The deadline for PC members to verify their eligibility to vote is Monday.

Confused about voting for the <a href="https://twitter.com/hashtag/PCPOLdr?src=hash&ref_src=twsrc%5Etfw">#PCPOLdr</a>? Code hasn't arrived? We can help!<br>Deadline for voter registration is Monday, March 5, at 11:59pm.<a href="https://twitter.com/hashtag/PCPO?src=hash&ref_src=twsrc%5Etfw">#PCPO</a> <a href="https://t.co/mAm1K3etTx">pic.twitter.com/mAm1K3etTx</a> —@TGranicAllen

"We're working hard to make sure everyone gets their verification codes by Monday," said Hartley Lefton, chair of the party's leadership election organizing committee, in a phone interview Thursday. "We've been very pleased by the tens of thousands of people who've been verified and who will vote to choose our new leader."

Party officials have not announced how many people signed up as members by the Feb. 16 deadline to be eligible to vote in the leadership race. They were to release that figure last Friday during a conference call, but cancelled it on short notice.

Multiple sources associated with different campaigns and the party brass tell CBC News that more than 190,000 members are eligible to vote.

Vic Fedeli, interim leader of the Ontario PC party, pegged the number of party members at 133,000 last month, but sources tell CBC News the membership has swollen during the leadership race to more than 190,000. (Mike Crawley/CBC)

Former leader Patrick Brown had claimed the PCs had amassed 200,000 members before he was forced to resign. A review ordered by interim leader Vic Fedeli shortly after Brown's January resignation weeded out invalid, expired and fake memberships. Fedeli put the party's actual number of memberships at 133,000.

However, membership has swollen again through the efforts of the leadership campaigns, putting the current number at more than 190,000, according to multiple sources.

Ranked ballot system

The voting system uses a ranked ballot: each party member will be asked to rank the candidates in order of preference. If no one takes 50 per cent of the votes on the first round, the candidate with the lowest number of votes is removed, and their supporters' second choices will be counted.

This means unless one candidate walks away with the leadership on the first ballot, party members' decisions on who they rank number two will prove crucial to determining the winner.

The system also gives equal weight to all 124 ridings in the province. Each riding counts for 100 points toward the overall count: the points awarded from each riding will be based on the percentage of the ballots cast for each candidate. That prevents a candidate from winning through signing up large numbers of members in a few populous ridings. Candidates will need to have broad-based support across the province to win.