White House Communications Director Bill Shine walks out after President Donald Trump to board the Marine One helicopter with him for travel to Mississippi from the White House in Washington, November 26, 2018.

Shine left Fox News after being embroiled in a sexual harassment scandal involving late Fox News Chairman Roger Ailes. While Shine was never accused of harassing women at the network, he was named in numerous lawsuits claiming he covered up Ailes' misconduct. Shine has denied wrongdoing. He joined President Donald Trump's administration as deputy chief of staff for communications in July.

Shine resigned from Fox in May 2017. The disclosure form shows he is due bonuses of about $3.5 million in 2018 and 2019. He also received $8.4 million in severance pay when he left the company.

In a letter obtained by CNBC, Sens. Elizabeth Warren, Richard Blumenthal, Sheldon Whitehouse and Edward Markey addressed what they say could be a breach of federal ethics laws after a new financial disclosure report showed Shine getting guaranteed bonuses from 21st Century Fox totaling approximately $7 million over this year and next.

Four Democratic senators are requesting the White House counsel's office to provide documents and answer detailed questions about whether Trump aide and former Fox News executive Bill Shine is breaking conflict of interest laws as he continues to get paid by 21st Century Fox.

Trump and Fox News have had a close relationship for several years. He was a regular guest on the network's morning show "Fox & Friends" and is friends with some on-air talent, including prime-time host Sean Hannity. Meanwhile, former top Trump aide Hope Hicks landed an executive spot at the company that will remain after the bulk of 21st Century Fox is merged into Disney next year.

The senators' letter is addressed to acting White House counsel Emmet Flood. In it, the lawmakers argue that the payments Shine received when he departed the network and the millions he continues to rake in may be in breach of ethic laws.

"21st Century's Fox's previous and ongoing payments to Mr. Shine of millions of dollars in severance payments raise questions about his compliance with federal conflict of interest statutes and regulations," they wrote.

The senators then asked questions that they described as a way to verify if he followed the regulations.

"Did you or other ethics officials at the White House provide Mr. Shine with any authorizations or ethics waivers regarding his participation in meetings or communications related to particular matters that affect 21st Century Fox, Fox News Channel, and its affiliates other than those identified in this letter," they asked.

White House Press Secretary Sarah Huckabee Sanders and Shine did not return a request for comment.

Read the letter to the White House.

The move to initiate an investigation comes as ethics experts publicly question whether Shine broke any laws after CNBC first reported last month on his continuing payments from Fox.

Walter Shaub, former director of the Office of Government Ethics, tweeted in November that Shine may have broken the "extraordinary payment regulation" which disallows federal employees to be paid in excess of $10,000 by a former employer, particularly if that company discovered an employee was being considered for a government position prior to their departure.

In their letter, the senators ask the White House whether Shine ever received the appropriate ethics waivers for not only these types of payments, but also his participation in meetings as it pertained to Fox News and 21st Century Fox.

The move to go after Shine also comes in the midst of Warren contemplating a run for president in 2020 and some within the Democratic Party questioning her decision to take a DNA test proving she had some form of Native American family origin. Warren took the test after Trump repeatedly mocked her as "Pocahontas" and questioned whether she had a Native American family background.

Jim Margolis, an advisor to former President Barack Obama during his 2008 and 2012 campaigns for president, told CNBC in an interview that he believes the timing of the test was ill advised because it coincided with the congressional midterm elections.

"I think her timing of doing it was really unfortunate," Margolis said. "I thought it would come post 2018 and I don't think it was helpful before the election. My guess is because it got so enmeshed in 2018 that she's going to probably need to come back around to it," he added.