Zurich - FIFA presidential favourites Gianni Infantino and Sheikh Salman bin Ebrahim Al-Khalifa embarked on a final scramble for votes on Wednesday ahead of the election to lead world football's scandal-tainted governing body.
The poll is seen as crucial to repairing the organisation's reputation, which has been battered by a series of unprecedented corruption scandals, but just two days before the vote, protests over the secrecy of ballots have mounted.
Front-runners Infantino - the Swiss-Italian acting chief of European football - and Sheikh Salman of Bahrain, Asian football's president, have so far stayed clear of the controversy over balloting.
Both were in FIFA's home city of Zurich trying to assemble the coalition of supporters needed to replace the disgraced Sepp Blatter and win the most powerful job in world football.
But rival candidate Prince Ali bin al Hussein of Jordan has appealed to the Court of Arbitration for Sport (CAS) to suspend Friday's election.
He has said transparent voting booths must be used to ensure that delegates do not take pictures of their ballots, a move required by certain regional confederations as proof of loyalty to a specific candidate, according Prince Ali.
"I want this election to occur as soon as possible, but not at the risk of undermining its integrity," the Jordanian royal said in a letter to FIFA's member associations.
Jerome Champagne of France, who along with South Africa's Tokyo Sexwale rounds out the list of five presidential challengers, has joined the Jordanian prince's call for greater transparency.
The Lausanne-based CAS has asked FIFA to respond to Prince Ali's concerns and said it will rule on his request for "urgent provisional measures" by Thursday, although it was not clear if the court could force a vote delay.
As closed-door campaigning continued at the luxury Zurich hotels favoured by football power-brokers, FIFA's top brass met for a final pre-election meeting at the body's headquarters on the outskirts of the city.
The executive committee is expected to finalise reform proposals to be presented for approval by the congress on Friday.
FIFA has said the reform package is crucial to cleaning out the graft that has plagued world football for decades.
Separately, a FIFA appeals committee is expected to rule on the pleas filed by Blatter and his one-time heir apparent Michel Platini, who are seeking to overturn the eight-year bans.
Both were punished in December over a $2 million payment that Blatter approved for the French football legend.
An appeals decision had been expected by Tuesday at the latest, and no reason has been given for the delay.
It was Platini's sudden and dramatic downfall that threw the race wide-open and led Infantino, his deputy at European confederation UEFA to enter the race.
While the hotly contested election remains too close to call, the two-thirds majority needed to secure a first-round victory seems unlikely for any candidate, meaning the poll could go to at least two rounds of voting.
Only a simple majority is required to win from the second round.
FIFA has 209 national associations, but currently only 207 are approved to vote, following the suspensions of Kuwait and Indonesia, although both countries could be cleared to participate on Friday.
The poll will take place nearly eight-months to the day after Swiss police stormed the plush Baur au Lac hotel in Zurich before dawn and arrested seven football executives indicted in United States for corruption.
Since then, 39 people within football and two companies have been charged by the US, Blatter and Platini have been banned, while FIFA's former secretary general Jerome Valcke has been sacked and thrown out of football for nine years over various corruption allegations.
Swiss prosecutors are also investigating FIFA's management and the attribution of the 2018 and 2022 World Cups to Russia and Qatar.