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: astonishingly well.

In the days leading up to Tuesday’s conclave, O’Malley has built up momentum. The archbishop of Boston, a baritone-voiced Capuchin Franciscan who prefers the order’s humble brown cassock, O’Malley has earned increasing attention from Vatican reporters, and, in the estimation of many experts, bypassed New York Archbishop Timothy Dolan as the prelate with the best, if still distant, shot of becoming the first American leader of the Roman Catholic Church.

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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 on Saturday, in a cafe by the Vatican. “Yeah that’s kind of an interesting storyline.”

For O’Malley, 68, it could also be a complicated one. The church has traditionally 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 unofficial ban, 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 expected 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 Joe Biden, he would probably play a major role in any Biden 2016 presidential bid. (Perhaps more conveniently, the brothers also have a sister, Donna, a nurse who is widely considered a saint.)

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 argued that recent concessions were inadequate.