Real Madrid presidential candidate Enrique Riquelme said on Sunday that rival Florentino Pérez has "crossed a red line" with his proposal to sell 5% of the club to an investor.
Club members will vote on Sunday to elect Madrid's next president, choosing between Pérez, 79, who has been in office since 2009, and renewable energy entrepreneur Riquelme, 37.
In an interview with the newspaper El País on Sunday, Pérez reaffirmed plans -- first trailed in a speech to members last year -- to allow an external investor to buy part of the member-owned club.
"If someone buys the smallest part, say 5%, that will simply help fix a value [of the club overall]," Pérez said.
"Florentino Pérez has crossed a red line," Riquelme responded, speaking to journalists later on Sunday. "I'll do the opposite. I'm announcing that this week I'll go to a notary to put in writing, in public, that I would never sell any part of Real Madrid to a third party, and I invite Florentino Pérez to accompany me.
"There are dozens of measures to increase income and reduce costs... The Florentino Pérez of 20 years ago would never have been open to a sale."
Madrid are one of four Spanish football clubs, alongside Barcelona, Athletic Club and Osasuna, which are run by their members rather than being public limited companies.
Pérez has argued that a change "which guarantees members are real owners of our club" is necessary.
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"We'll continue being a members' club, but we would create a subsidiary, owned by the members," he said in a speech to Madrid's annual members' assembly last November. "We'll allow 5% for a partner, if someone is willing to invest a significant amount, for a symbolic participation. Those investors should always respect our values."
Pérez and Riquelme both attended Madrid's women's team's 2-0 win over Granada in Liga F on Sunday.
This week's elections are the first to be held at the club since 2006, with Pérez subsequently elected uncontested, with no need for a vote, in 2009, 2013, 2017, 2021 and 2025.