The strategist was also at the December party. Bannon's planning and Mercer's money were among the key drivers of Trump's success.
Trump, Mercer and Bannon were enjoying a moment of victory.
"The Mercers laid the groundwork for the Trump revolution," Bannon told the New Yorker's Jane Mayer last March. "Irrefutably, when you look at the donors during the past four years, they have had the single biggest impact of anybody, including the Kochs."
But the once-close bond between candidate, strategist and mega-donors was torched this week.
On Wednesday, excerpts of Michael Wolff's new book on the administration hit the news cycle, including explosive comments Bannon reportedly made about Trump's family.
The president responded with a blistering attack on his former adviser, who left the White House last August and returned to Breitbart News as chairman.
"Steve Bannon has nothing to do with me or my presidency," the president said Wednesday.
"When he was fired, he not only lost his job, he lost his mind."
Thursday night he called Bannon "Sloppy Steve" in a tweet.
The rift between the two deepened Thursday, when the notoriously press-shy Rebekah Mercer — whom Newsmax Media owner Christopher Ruddy has admiringly called "the First Lady of the alt-right" — publicly rebuked Bannon and stood by the president in a statement.
"I support President Trump and the platform upon which he was elected," Mercer said. "My family and I have not communicated with Steve Bannon in many months and have provided no financial support to his political agenda, nor do we support his recent actions and statements."
And although Rebekah Mercer remains a stakeholder in Breitbart News, the significance of her public slap at Bannon can't be overstated.
Long before Trump stepped onto the campaign trail, the Mercer family and Bannon were drawing up schematics for an outsider populist presidential candidate who could topple the establishment. It was a dream years in the making. And now Trump, who fulfilled their hopes, has come between Mercer and Bannon, a further escalation of the conflict ripping through the president's base.
Robert and Rebekah Mercer's rise as Republican power brokers was unique.
Robert Mercer is a former IBM computer scientist who made billions later in life by applying complex programming techniques to financial trading as the co-CEO of Renaissance Technologies, Bloomberg reported.
Quiet and socially awkward — he once told a friend he preferred the company of cats to people, according to the Wall Street Journal — Mercer has an extreme view on small government and wealth.
"Bob believes that human beings have no inherent value other than how much money they make," a colleague told the New Yorker.
"If someone is on welfare they have negative value. If he earns a thousand times more than a schoolteacher, then he's a thousand times more valuable."
Rebekah, the middle of the Mercer's three daughters, studied biology and math at Stanford, and earned a master's degree there in management science and engineering, The Post has reported.
She worked for a brief period at her father's hedge fund, then raised four children with her husband, a French-born Morgan Stanley managing director. The couple live in a $28 million apartment on Manhattan's Upper West Side, according to Bloomberg.
As her family became more involved in conservative causes, Rebekah took the reins of the family foundation. According to an analysis by The Post, the foundation put $35 million into right-wing think tanks and policy groups between 2009 and 2014.
The Mercers' journey to the White House began in 2011. It hinges on a Jimmy Stewart character.
In 2011, the future president first met Bannon to discuss a possible presidential run. The same year, the Mercers were introduced to conservative flamethrower Andrew Breitbart and Bannon, agreeing to invest $10 million into Breitbart News, according to the New Yorker.
One stipulation of the Mercers' deal was to place Bannon on the news company's board. Breitbart died months later, which left the news operation in Bannon's hands.
The key figure in the Bannon-Trump-Mercer White House success is Patrick Caddell, a former Democratic pollster. If the Mercers were the money, Bannon the strategist and Trump the candidate, Caddell was the player who first wrote the instruction manual.
For years, the pollster conducted extensive public opinion research showing Americans were sick of the two-party system and "ripe for an outsider candidate to take the White House," Mayer wrote in the New Yorker.
The longtime political operative would eventually term this the Candidate Smith project, a reference to "Mr Smith Goes to Washington," a 1939 Jimmy Stewart film about an outsider political candidate.
The data increasingly showed "mounting anger toward wealthy elites, who many Americans believed had corrupted the government so that it served only their interests," Mayer wrote. By 2012, Caddell was sharing the populist trend in the numbers with Bannon. The data began to shape Bannon, and Breitbart's, worldview.
The same year, the Mercers became fed up with the Republican establishment after President Barack Obama's reelection. After Republican candidate Mitt Romney's loss, Rebekah Mercer chewed out a room full of party donors at a meeting at New York's University Club for bungling the election.
In 2013, Caddell showed his polling data to both Robert Mercer and Bannon at a conservative conference in Palm Beach. The donor was so intrigued with the populist trend he would keep Caddell polling data right up to the 2016 election. The results eventually found their way to Roger Stone, who shared them with his longtime confidant, Trump. As the Republican presidential primary season started, Caddell tested the field as potential "Mr Smith" upstarts.
"People didn't think Trump had the temperament to be President," Caddell told Mayer. "He clearly wasn't the best Smith, but he was the only Smith."
The Mercers, born number-crunchers, were impressed with Caddell's work. When their initial preferred presidential candidate — Ted Cruz — left the campaign trail, the family switched to Trump. And when Trump's campaign lagged in the middle of the 2016 campaign, the Mercers were quick to push their own associates — Bannon and Conway — into the campaign's powers structure.
