Senate Minority Leader Charles Schumer Chuck SchumerPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' 3 reasons why Biden is misreading the politics of court packing Cruz blocks amended resolution honoring Ginsburg over language about her dying wish MORE (D-N.Y.) is pushing the Pentagon for details on how it is protecting officials from retaliation when they testify as part of the impeachment inquiry into President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE.

Schumer sent a letter to Defense Secretary Mark Esper on Monday asking for details on how the department is protecting two officials set to testify this week, as well as requesting the Pentagon formally notify personnel about their ability to share information with Congress.

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"I believe the Department of Defense must do more to formally ensure that all Department military and civilian personnel understand that they may make protected disclosures to Congress free from retaliation," Schumer wrote in the letter.

Schumer, in his letter to Esper, also asks the Defense Department to "immediately cease any efforts to prevent officials from cooperating with Congress" and hand over requested documents related to the Ukraine Security Assistance Initiative, the program at the heart of the impeachment inquiry.

He also wants a briefing on "what actions are being taken to ensure that [Lieutenant Colonel] Vindman, Ms. Cooper, and other whistleblowers like them are afforded appropriate protections — both from workplace reprisals and for their personal safety and that of their families."

Both Alexander Vindman Alexander VindmanImpeachment witness Alexander Vindman calls Trump Putin's 'useful idiot' The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Strzok: Trump behaving like an authoritarian MORE, the leading Ukraine expert on the National Security Council, and Laura Cooper, the deputy assistant secretary of Defense for Russian, Ukrainian and Eurasian affairs, are expected to testify publicly this week as part of the second round of impeachment inquiry hearings.

The House is weeks into its investigation into whether Trump tied Ukraine aid to the country opening up an investigation into former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE and his son Hunter Biden. The House Intelligence Committee is set to have three days of public hearings this week, with a total of eight individuals testifying.

Trump has lashed out at Vindman, describing him as a "Never Trumper" — comments that sparked bipartisan backlash on Capitol Hill.

Schumer, in his letter to Esper, praised Vindman, Cooper and whistleblowers more broadly.

"They are patriots for being willing to do what we hope and expect all civilian and military officials will do when asked: tell the truth. It is incumbent on you to ensure that they and others who come forward are afforded the same protections as whistleblowers and are protected from retaliation," he added.