From left, siblings Ivanka, Eric and Donald Trump, Jr., together in Trump Tower in New York in June 2012. (Jennifer S. Altman/FOR THE WASHINGTON POST)

On the 26th floor of Trump Tower in Manhattan, Secret Service agents stand guard at the elevator to keep all but a handful of people away from Republican presidential candidate Donald Trump. But on this spring afternoon, Trump’s son-in-law, Jared Kushner, breezes right through to Trump’s corner office, not long after his oldest son just left.

“Hi, Jared. Are you all set with the meetings?” Trump said to Kushner, a stack of papers in his hand.

One trademark of the most unconventional campaign in modern history is that members of Trump’s family — who have virtually no political experience — are so deeply involved in his campaign that they often act as gatekeepers and strategists. Their influence was clear this week when Trump decided to fire his campaign manager, Corey Lewandowski, after his adult children and Kushner met with him on Father’s Day at a Trump golf course in New Jersey.

Kushner, who is married to Ivanka Trump, and Donald Trump’s three oldest children are in frequent contact with Republican National Committee officials, who have come to expect at least one of them at campaign meetings. Family members are involved in drafting speeches, messaging and policy, several people familiar with the campaign said.

Family members are often among the closest advisers to any candidate, of course. Kevin Madden, a veteran political operative who advised 2012 GOP nominee Mitt Romney, recalled how Romney’s wife and sons spoke on the candidate’s behalf, adding personal details that others could not. But, he added, “the family members in the Romney campaign deferred to professional staff” in the running of the campaign.

Republican presidential candidate Donald Trump called his sons Don and Eric up on stage during a rally in January. "They're good boys," Trump said after they spoke in support of their father. (Reuters)

Trump, however, has never played by the book. And because his operation is so tiny — about 70 paid staff members compared with Hillary Clinton’s 683 — his family’s impact is even more pronounced.

The campaign’s latest financial filings showed that not only did Trump have a surprisingly small amount of cash on hand — $1.3 million — but he had spent more than $1.1 million in May reimbursing his properties and family members for expenses.

Even as they help on the campaign, Trump’s oldest children are also shouldering more day-to-day responsibilities at the Trump Organization, where each is a top executive. Don Jr., Ivanka and Eric Trump — all in their 30s — have offices one floor below their father’s in Trump Tower, the Manhattan landmark that houses both the headquarters of the campaign and the family business.

“Everything we’ve ever done, we’ve done as a family,” Eric Trump, 32, said in an interview this year. “Every project we’ve ever built, we’ve built as a family. ‘The Apprentice’ we did as a family. The golf courses and hotels we do as a family. . . . We vacation as a family.”

Don Jr., 38, spoke to ABC’s “Good Morning America” on Tuesday to explain why Lewandowski was let go. “There does have to be a transition to the general” election and it was “time to move on,” he said.

Don Jr. also fielded questions about the timing of his father’s vice presidential deliberations, poor fundraising numbers and advertising strategy.

Lewandowski’s abrupt departure was spurred in part by suspicions within the family that he was trying to sideline Kushner by spreading dirt to the media about him.

In 2005, Kushner’s father, Charles, was convicted in federal court of making illegal campaign contributions, tax evasion and witness tampering. The charges were rooted in a nasty family dispute that involved Charles Kushner hiring a prostitute to seduce his brother-in-law in order to get an incriminating sex tape.

Jared Kushner — the 35-year-old chief executive of a huge family real estate company — now sits in campaign meetings with the prosecutor in that case, New Jersey Gov. Chris Christie (R), who was a U.S. attorney at the time and now advises Trump.

Kushner is as low-key as his father-in-law is boisterous. He avoids making public comments but is widely credited with helping Trump write his well-received March speech to the American-Israel Public Affairs Committee, or AIPAC. In it, Trump used a teleprompter for the first time.

Kushner looks younger than his 35 years and is the father of three small children with Ivanka. He comes from an Orthodox Jewish family; Ivanka converted before they married. Kushner also has made a long list of donations to Democrats in New York and New Jersey.

But he is an enthusiastic supporter of Trump and frequently travels with him, including a recent trip to Washington to meet with House Speaker Paul D. Ryan’s policy team. Kushner, who is also publisher of the New York Observer newspaper, helped connect Trump to former secretary of state Henry Kissinger for foreign affairs advice and to Rupert Murdoch to mend relations with his Fox News Channel.

During a victory speech in Indiana during the primary, Trump praised his son-in-law, telling the crowd: “Honestly, Jared is a very successful real estate person, but I actually think he likes politics more than he likes real estate. . . . He’s very good at politics.”

In the April encounter in Trump Tower, Kushner deferred questions about Trump to his wife. But he did say that his father-in-law called Ivanka three times a day when she couldn’t join him on the campaign trail because she was about to have a baby.

RNC Chairman Reince Priebus speaks to Kushner and the three eldest Trump children regularly in these weeks ahead of the party convention in Cleveland, according to RNC spokesman Sean Spicer. Trump’s family is also expected to play a central role at the event.

Ivanka, 34, who is as controlled as Trump is spontaneous, has urged her father to soften his tone, several people who know the family said. She has tried to avoid commenting on her father’s specific policies publicly and focused her huge social media following on noncontroversial topics such as balancing work and children.

She has also defended her father when critics have called him sexist, saying he always promoted women to top positions and offered her the same chance in the workplace as her brothers.

One person familiar with the campaign operation, who requested anonymity to discuss the Trump family, said the mogul’s children have been involved in part because “for better or worse, the Trump brand is intermingled with the campaign.” He said the children help Trump understand which of his messages are getting across and which are flopping, and that Ivanka was key in him offering support for Planned Parenthood’s non-abortion work, even though many Republicans do not.

In addition to the three oldest children, from his marriage to Ivana Trump, Donald Trump has a 22-year-old daughter, Tiffany, from his second marriage, to Marla Maples, and a 10-year-old son, Barron, with current wife, Melania Trump.

“Good kids. Good relationships,” Trump said, when asked about his children in a recent interview. He said that relationships tightened when they were old enough to start working with him. People often get the wrong idea of who he really is, Trump said, “but people do think I have been a good father.”

Jay Goldberg, a top New York lawyer who advised Trump for 15 years until 2005, said Trump is “very loyal to his children” and is lucky that “they are monumentally smart.”

Goldberg said his own work with Trump waned when the children, though then only in their 20s, had strong ideas and the backing of their father about how to expand the family business. Trump’s core business was real estate holdings and they wanted to expand, selling the Trump name to other hoteliers in franchise deals as well as marketing Trump merchandise ranging from furniture to fragrances.

“They were interested in franchising, I was not,” said Goldberg. “I didn’t like the suits and cuff-links and shirts.”

At a time when Trump is under attack even by many within his own party, several who know him say a man who has always valued family loyalty is drawing them even closer.

“We’re naturally protective of our father. I think we also have a unique ability — a lifetime of trust with him,” Eric Trump said in an interview on Tuesday. “We have 11 years of being executives within his organization. . . . He’s learned to trust us. The one thing that I can tell you is Ivanka, Don and I will be by his side. ”

Dan Zak and Alice Crites contributed to this report.