ROME — As he prepares to declare two revered 20th-century pontiffs as saints, Pope Francis once again proved that he rivals John XXIII and John Paul II in terms of star power by persuading a renowned anti-clerical politician to suspend a hunger and thirst strike that doctors and friends believed was putting his life in jeopardy.

Marco Pannella, the 83-year-old leader of Italy’s Radical Party, had refused to eat or drink liquids to protest what he sees as inhumane conditions in Italy’s overcrowded prison system. Pannella refused to suspend the strike despite undergoing an operation earlier in the week for an aneurysm in his aorta, even declining to accept blood transfusions.

On Friday afternoon, Pope Francis phoned Pannella at Rome’s Gemelli hospital, with the two men speaking for roughly 20 minutes. Afterwards, Pannella said that out of respect for the pope, he had agreed to drink some coffee and to accept two blood transfusions that medical personnel urged him to undergo.

According to a transcript of the call released by the Radical Party’s radio station, Francis vowed that he would help Pannella “fight against” the conditions facing the prisoners, and urged the elderly politician to “be courageous.”

Sources told the Globe that the conversation between Francis and Pannella was arranged by a longtime leader of the Radical Party, former Italian foreign minister Emma Bonino, who called the pope’s residence Thursday to ask for help. Bonino spoke with Francis, among other things expressing fear that if Pannella continued to refuse liquids he might be risking his life.

A Vatican spokesperson neither confirmed nor denied the call, citing a policy of not commenting on the pope’s personal conversations. However, the spokesperson noted that Pannella has previously expressed esteem for Francis and especially the pontiff’s visit last year to a youth prison in Rome to wash the feet of inmates.

Francis’s gesture is all the more remarkable given that the Radical Party, and Pannella in particular, have long battled the Church on a wide range of issues, from divorce and abortion to seeking to abolish the tax and legal privileges afforded the church under a 1984 treaty with the government.

Pannella is a self-described atheist and bisexual, although he has voiced appreciation over the years for social justice positions of recent popes.

This is not the first time Francis has helped to end a hunger strike concerning prison conditions.

Over Christmas, a group of immigrants from Tunisia and Morocco being held at a detention center in Rome had sewn their lips shut and refused to eat, protesting what they described as degrading and inhuman treatment.

Several of the immigrants were also sleeping outside as part of their protest, despite freezing conditions during the Roman winter.

Although the protestors were not Catholics, a local priest visited them and vowed to carry a letter outlining their concerns to Francis, assuring them the pontiff would take an interest. That was enough to persuade the immigrants to suspend their hunger strike. Medical personnel were summoned to remove the stitches from the immigrants’s mouths, and a small meal was organized.

On Sunday, Francis will preside over a canonization ceremony, the formal term for declaring someone a saint, for John XXIII and John Paul II. Officials expect more than 1 million people to attend, making it the largest public event in Rome since the funeral of John Paul II in April 2005.