It was Mr. Flaherty who organized a confidential meeting between Mr. Romney and dozens of conservative leaders in Washington last week. It was Mr. Flaherty who persuaded five former ambassadors to the Vatican to publicly back Mr. Romney over his Catholic rivals, Mr. Santorum and Newt Gingrich, before the New Hampshire primary. And it is Mr. Flaherty who has coached Mr. Romney on how to defend his position on fraught issues like abortion during debates, according to campaign colleagues and conservative leaders.

In a field that churns out hard-charging operatives, Mr. Flaherty, 46, is something of an anomaly, described by those he has courted as disarmingly mild-mannered, soft-spoken and philosophical. But they know him, too, as a man of deep feeling, propelled into the law by the murder of an aunt at a Boston church, where he returned to pray during jury deliberations as a prosecutor. He tends to begin conversations with an update on his three sons and two brothers, whom he speaks to several times a day.

He deliberately avoids the hard sell, to the point where some conservatives say the Romney campaign has not been aggressive enough. Mr. Flaherty is known to tell skeptical evangelical leaders and reluctant conservative luminaries backing rival candidates that he can live with being their second choice in the nomination contest. “Keep us in mind,” is a common Flaherty refrain, said those who have spoken with him, followed by this: “Our door is always open.”

That style has earned him a deep reservoir of good will, even among those who disapprove of Mr. Romney. Several conservatives said he is the only campaign official they will work with.

His highly personal approach has paid dividends. As 150 Christian conservative leaders prepared to meet in Texas a few weeks ago to discuss ways to block Mr. Romney’s path to the nomination, one of the attendees, Gary L. Bauer, felt compelled to call Mr. Flaherty and offer a personal assurance: he would not allow the meeting to dissolve into a Romney bashing session.