WASHINGTON (Reuters) - In what is likely to be their first public use of subpoena power since taking over the U.S. House of Representatives in January, Democrats were set to vote on Tuesday on subpoenaing documents on the Trump administration’s migrant family separation policy.

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If approved, the subpoenas by the House Oversight Committee would show Democrats beginning to invoke the investigative clout they obtained when voters in November handed them majority control of the House and took it away from Republicans.

Democrats are initiating a series of investigations of President Donald Trump, his personal finances and business interests and his 25-month-old administration, including controversial policies such as family separation.

The House Oversight Committee will vote on a resolution from Chairman Elijah Cummings to issue subpoenas to Homeland Security Secretary Kirstjen Nielsen, Attorney General William Barr, and Health and Human Services Secretary Alex Azar, according to committee documents reviewed by Reuters.

The resolution is likely to pass as the committee’s Democrats outnumber Republicans, 24-18.

As part of a crackdown on illegal immigration, the Trump administration separated thousands of children from their immigrant parents who crossed from Mexico into the United States, placing many in detention camps.

Immigration advocates and Democratic lawmakers say many traumatized children, having fled their home countries, were held in institutionalized settings for too long under the policy. The Department of Health and Human Services’ inspector general said in a January report that the administration began ramping up separations in 2017. They accelerated in 2018 after Trump implemented a “zero tolerance” policy to criminally prosecute and jail all illegal border crossers.

Outrage over the policy led Trump to sign an executive order on June 20, 2018, reversing course.

In a Feb. 22 letter to Cummings, top Oversight Committee Republican Jim Jordan said the three departments “have been working expeditiously” to provide documents and information to the panel and that it was premature to vote on subpoenas.

An Oversight Committee official said the documents to be subpoenaed are the same as those requested from the departments in a July 2018 letter signed by Cummings, when the Democrats were in the minority, and Republican Representative Mark Meadows.

After the departments refused to hand over the documents, the panel’s then-Republican chairman Trey Gowdy declined Cummings’ request to issue subpoenas and refused to hold a vote, said the official, speaking on condition of anonymity.