VATICAN CITY – Shortly after arriving in Rome with his boss, dark horse papal contender Cardinal Sean O’Malley, Terrence Donilon received an email. It was from his brother Tom, the national security adviser to President Obama.

“How’s it going?” the top White House official asked.

The answer is: Very well. The archbishop of Boston, a baritone-voiced Capuchin Franciscan who prefers the order’s humble brown robe, O’Malley, 68, has, in the estimation of many experts, is the prelate with the best, if still distant, shot of becoming the first American leader of the Roman Catholic Church.

That unexpected buzz has also raised a remarkable possibility for Terry Donilon, O’Malley’s communications director and cabinet member.

One Donilon brother “working for the most powerful man on the planet, and the other one could work for the most powerful religious leader on the planet?” mused Terry, dressed in blue baseball cap and polo shirt in a cafe by the Vatican.

“Yeah, that’s kind of an interesting storyline.”

The church has excluded Americans from papal consideration for fear of allotting too much influence and might to the world’s superpower. Changes in geopolitics and the demands of the church have softened that, but it’s unclear what it would mean for a potential symbol of world peace to have a close associate who is also the brother of an architect of Obama’s foreign policy.

And while reports have emerged that Tom Donilon is likely to step down this year (“I’m still fully engaged,” he said), there is also a third Donilon brother, Mike. A longtime political aide to Vice President Biden, he would probably play a major role in a Biden 2016 presidential bid.

“This is an extraordinary circumstance, not one that we’ve ever thought about,” Tom said.

In the unlikely event that any of the cardinal electors are concerned by the Donilon family ties, it is also worth noting that Terry Donilon, 52, has already shown his willingness to stand up to the Obama administration.

Donilon attacked provisions in the administration’s Affordable Care Act that required health insurance plans to offer contraceptives and access to other procedures anathema to the Catholic Church. He said recent concessions were inadequate.

“The way the administration handled that was poor,” Donilon said, adding, “My brothers have their life and their careers, and I’ve had my life and my careers, and if they intersect at times because of issues, so be it.”

The Donilon children grew up in a solidly Irish section of Providence, R.I., where St. Michael’s Parish acted as a center of gravity. Faith, Terry said, was the “fabric of who we were.”

Terry studied theater education at Emerson College and then worked in radio, where an executive tried to convince him to change his name to Sandy Beach.

In the mid-1980s, one of the radio show’s guests was Providence Mayor Joe Paolino, an old high school pal of Tom’s. The elder Donilon had already gone on to distinguish himself as a White House staffer and one of the Democratic Party’s sharpest political minds.

“Come see me,” Paolino told him. Donilon went to work for Paolino in the mayor’s office and then on his unsuccessful campaign for governor. Then he worked in the office of Gov. Bruce Sundlun and on his unsuccessful re-election campaign. He subsequently worked for Robert Weygand, a member of Congress who lost his bid for a Senate seat.

“My candidates kept losing,” Terry, 52, said. He became press director for a supermarket chain.

In 2005, the Boston archdiocese, with the help of a group close to Biden, began a search for a new communications director.

Donilon said his interview with O’Malley was going terribly because his beeper kept buzzing and his cellphone kept ringing because of a meat recall by the supermarket chain.

Luckily for Donilon, the conversation turned to a mutual acquaintance, former Providence mayor Buddy Cianci.

“I called Tom when I got offered the job,” Terry said, adding that his brother told him, “Don’t be hobnobbing around Boston because you work for an archbishop.”

O’Malley presided over a diocese that, under Cardinal Bernard Law, had become ground zero of the priest sex abuse scandal.

“We were under assault and by our own doing,” said Terry, a divorced father of two. “This wasn’t like an unprovoked attack.”

But Terry said O’Malley helped heal the diocese and restored trust. When Terry Donilon talks about his boss, his political instincts kick in, and he sounds like any operative boosting his candidate.

He called the cardinal a brilliant messenger who took the diocese “out of the fiscal abyss” and “provided the right support for victims of sexual abuse.”

Donilon acknowledged that the church had a strong political dimension, recounting a memorable exchange with a priest in Boston.

“This place is driving me nuts, I got out of politics to avoid all this,” Donilon said he told the cleric. “Terry,” he said the priest responded, “we invented politics 2,000 years ago.”

And in Rome, he has found himself at the height of campaign season.

On Sunday, he accompanied O’Malley to Santa Maria della Vittoria, the cardinal’s titular church. Donilon paused from wrangling dozens of international journalists and paused to look out with a stunned expression at the packed pews.

The Rev. Rocco Visca introduced O’Malley by saying, “May this be your last visit here as cardinal, and may we be the first church you visit as the next pope.”

The American prelate offered a homily about bringing strayed sheep back into the flock, prayed in Italian that impressed some of the Italian reporters in the back of the church and then exited in a procession of priests. Donilon, backpack on his shoulder, brought up the rear.

Outside, a group of Italian tourists joked about all the company they would have if the cardinals chose O’Malley.

“All of America will be here,” Domenico Calocero said to his friend. “Obama would fly here the day after.”