The "soothing, whispery voice" of Donald Trump's son-in-law could end up counselling the president-elect in a top White House advisory role — a prospect that would challenge the federal U.S. anti-nepotism laws.

Jared Kushner, the 35-year-old whose quiet manner was described by the New York Times as a comforting presence for Trump during his final election push, is reportedly up for an influential post in the new administration.

If appointed, however, Kushner's staff job in the administration could be illegal, according to government ethics attorneys.

That's because it would likely run afoul of a 1967 bill signed by former president Lyndon B. Johnson to prevent all-in-the-family type governance.

"We don't have royalty in America," says Richard Painter, the former chief ethics lawyer for president George W. Bush. "The president should not be coming in with family members in top positions in the White House.

"You just don't use your position as president of the United States to entrench a family member to a fairly high-ranking position in the United States government."

Unclear if White House is federal 'agency'

The passage of the anti-nepotism bill by Johnson was seen as a response to his predecessor, John F. Kennedy, tapping his younger brother Bobby for attorney general, as well as the apparent sidelining of Johnson in his former duties as Kennedy's vice-president.

Under Title 5, Section 3110 of the U.S. Code, a "public official," such as president, "may not appoint, employ, promote, advance or advocate for" anybody who is a "relative." The provisions list "son-in-law" as falling under the definition.

Painter expects a quick court challenge invoking Title 5 if Trump tries to name Kushner to a full-time White House position. But there may still be a way to circumvent the statute, as it only covers jobs in federal agencies.

Whether that definition is broad enough to cover the White House is unclear.

Veteran congressional ethics lawyer Stan Brand notes that the White House is not considered a federal agency under the Freedom of Information Act, for example.

Kushner, shown leaving Trump Tower in New York, is considered an influential member of Trump's inner circle. (Carolyn Kaster/Associated Press)

For Kushner, a larger legal problem may be tied to his finances. Trump's tapping of Kushner would be perceived as enabling him "to advance his business interest by knowing things that he otherwise wouldn't and shouldn't know," Brand says.

Kushner, who is married to Trump's daughter Ivanka, owns the New York Observer newspaper.

Trump has already taken the step of requesting top-secret clearance for Kushner, according to reports. (Though the president-elect's transition team denies the reports.)

Alternatively, Trump might be able to stretch the rules by appointing Kushner to a special presidential commission.

U.S. media reports say Trump has requested security clearance for Kushner, though the president-elect's transition team denies those claims. (Mary Altaffer/Associated Press)

"If the president were to want to have a relative on one of those boards, that's a part-time position. They don't receive a salary, as these are non-compensated boards," says Painter.

The Clinton connection

There is some precedence for this, he adds, noting that Hillary Clinton was appointed to chair a 1993 task force on health-care reform by her husband, former president Bill Clinton.

Clinton would not have collected a salary.

"She was essentially a volunteer," says James Thurber, a political science professor at American University who writes about lobbying and ethics.

Similarly, Kushner might waive any salary and could end up on a commission such as the President's Intelligence Advisory Board.

Commissions authorized by Congress would be paid for by Congress, whereas commissions created by the president would likely have operational costs taken out of the White House budget.

The president is supposed to be a champion of the rule of law. Does he really want to launch his presidency with an arguably illegal act? - Norm Eisner , former counsel for ethics in Obama's administration

Presidents can establish commissions through executive order.

Kushner is reportedly consulting with lawyers over whether forgoing a salary and putting his investments into a blind trust would give him a way to have Trump's ear in a formal capacity.

Norm Eisen, a former chief ethics lawyer in President Barack Obama's administration, doubts that tactic will fly. He cites an interpretation of the Antideficiency Act, a law that deems it a felony for employees in the executive branch to work for free when Congress hasn't appropriated their salaries.

In other words, Eisen explains, "the president can't accept free consulting services."

Kushner and his wife, Ivanka Trump, pose for a photo at a gala in New York. Trump has publicly praised his son-in-law, saying 'he's very good at politics.' (Evan Agostini/Associated Press)

His advice? "I would just tell the president 'don't do it'" — particularly at a time when so much scrutiny has been placed on Trump's decision-making.

Adding a layer of uncertainty about whether Trump is breaking the law would be unnecessarily distracting, Eisen says. "The president is supposed to be a champion of the rule of law. Does he really want to launch his presidency with an arguably illegal act?"

Even without a formal title, Brand says, there's little that can be done to stop Kushner from advising the president outside government — so long as Trump is willing to pick up the phone.

"He's the president," Brand says. "And the president can still get advice from whomever he wants."