Last week, as Donald J. Trump endured one of the most tumultuous stretches of his presidential campaign, a few longtime allies in New York conservative circles met for dinner and a drink. As the evening progressed, the conversation turned to an inevitable topic: What would it take to give Mr. Trump his best shot at winning?
A few days later, one of the guests, Stephen K. Bannon, the executive chairman of Breitbart News, would become Mr. Trump’s campaign chief in a sudden shake-up. But it was a guest without a formal role in the campaign, a conservative philanthropist named Rebekah Mercer, who has now become one of its most potent forces.
Mr. Bannon’s ascension on Wednesday, urged on Mr. Trump by Ms. Mercer among others, shows how a cadre of strategists, “super PACs,” and political organizations quietly nurtured by her family have emerged to play a pivotal role in Mr. Trump’s presidential campaign.
Over more than half a decade, Ms. Mercer’s father, the New York investor Robert Mercer, has carved an idiosyncratic path through conservative politics, spending tens of millions of dollars to outflank his own party’s consultant class and unnerve its established powers. His fortune has financed think tanks and insurgent candidates, super PACs and media watchdogs, lobbying groups and grass-roots organizations.
Many of them are now connected, one way or another, to Mr. Trump’s presidential bid. Mr. Trump’s new campaign manager, Kellyanne Conway, is a veteran Republican pollster who previously oversaw a super PAC financed by the Mercers. Mr. Bannon oversaw Breitbart, an outlet that has often amplified Mr. Trump’s message and attacked his perceived enemies. Mr. Mercer reportedly invested $10 million in the media company several years ago, and most likely still has a stake: a company sharing an address with Renaissance Technologies, the hedge fund Mr. Mercer helps lead, remains an investor in Breitbart, according to corporate documents filed in Delaware.
Mr. Trump is also relying on Cambridge Analytica, a voter data firm backed by Mr. Mercer, whose staff members are working with Mr. Trump’s vendors to identify potential Trump supporters in the electorate, particularly among infrequent voters. A Mercer-backed super PAC supporting Mr. Trump is now being shepherded by David Bossie, a conservative activist whose own projects have been funded in part by the Mercers’ family foundation, according to tax documents.
Mr. Bannon has worked particularly closely with the family in recent years.
“I think they have complete confidence, and rightly so, in Steve Bannon’s decisions and what he brings to the table politically,” Mr. Bossie said. “He has been smart and successful in running these different political operations. And those things have come to the Mercers’ attention.”
The Mercers, who rarely grant interviews, declined through a spokesman to comment. Mr. Mercer, 70, a mathematician and competitive poker player who spent his early career at I.B.M., joined Renaissance in the 1990s and rose to become the co-chief executive officer, earning hundreds of millions of dollars along the way. Today he and his wife, Diana, live on a sprawling estate on Long Island’s North Shore where, according to court records, he installed a $2.7 million model railroad set (and later sued the vendor for overcharging him).
Like many elite donors, the Mercers shun mainstream media attention — even while financing alternative outlets that provide content for conservative activists. That includes not just Breitbart, but the self-described watchdog organization Media Research Center as well as the Government Accountability Institute, home to Peter Schweizer, the author of “Clinton Cash,” a book examining the Clinton family philanthropies. (Ms. Bannon co-founded the institute and Ms. Mercer, 42, has served on its board; she is also co-produced a documentary based on the book and released last month, just ahead of the Democratic National Convention.)
Their giving includes Libertarian organizations, such as the Cato Institute, and political organizations like the Club for Growth, which spends millions of dollars each election cycle in Republican primaries, hoping to promote orthodox conservative policies on taxes and spending. The Mercers are also significant donors to the sprawling political network overseen by the political activists Charles and David Koch, which is also Libertarian-leaning.
But unlike the Koch brothers, who remained neutral in the Republican primary and have said their organizations will focus on congressional races this fall, the Mercers were deeply involved in the Republican nominating battle this year. And they have a shown a taste for more bare-knuckled and populist politics than most of Mr. Mercer’s fellow hedge fund magnates.
The family originally backed Senator Ted Cruz of Texas, a more traditional conservative but one who, like Mr. Trump, is disliked by much of the party establishment. During the early phase of the campaign Mr. Mercer donated $13 million to a super PAC supporting Mr. Cruz. In doing so, he broke with many peers in the elite donor world, who looked to candidates like Jeb Bush or Senator Marco Rubio of Florida.
The Mercers maintained close control over the group’s purse strings, installing Ms. Conway to oversee the group and coordinate with several other pro-Cruz groups, an unusual move for a super PAC. During the Republican primary, the group ran ads questioning Mr. Trump’s conservative credentials, hoping to outflank Mr. Trump.
But the Mercers moved to support Mr. Trump after he won the nomination. They were helped in part, according to a person who asked for anonymity to describe the family’s thinking, by Mr. Trump’s growing emphasis on traditional conservative ideas such as tax cuts. And the family broke with Mr. Cruz in highly public fashion after his speech at the Republican convention, when the Texas senator refused to endorse Mr. Trump and instead suggested that Republicans should “vote your conscience” for candidates “up and down the ticket.”
In an extraordinary rebuke, the Mercers issued a rare public statement, calling themselves “profoundly disappointed” in Mr. Cruz for refusing to endorse Mr. Trump.
In late June, the Mercer-financed super PAC quietly re-formed as Make America Number One, now a pro-Trump entity. Mr. Bossie, a longtime conservative activist who has produced documentaries about the Clinton family and illegal immigration, is now leading the group, which is likely to raise more money from the Mercers to pay for attacks on Hillary Clinton.