Sens. Elizabeth Warren Elizabeth WarrenHarris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda Judd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report MORE (D-Mass.) and Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE (D-N.Y.) are among a group of six Senate Democrats calling on the Department of Defense to explain whether deployments to the southern border are negatively affecting the combat readiness of troops.

In a letter sent Monday to acting Defense Secretary Patrick Shanahan Patrick Michael ShanahanHouse Armed Services chairman expresses confidence in Esper amid aircraft carrier coronavirus crisis Boeing pleads for bailout under weight of coronavirus, 737 fallout Esper's chief of staff to depart at end of January MORE, the senators wrote that they are "extraordinarily distressed" by concerns raised by Marine Corps Gen. Robert Neller regarding the effect of deployment to the border on readiness.

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Neller wrote in internal memos in February and March that deployments to the southern border and funding transfers under President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE's declaration of a national emergency were posing an "unacceptable risk to Marine Corps combat readiness and solvency," according to the Los Angeles Times, which obtained copies of the memos.

Neller added in the memos that he had canceled or reduced military training in at least five countries because of the president's “unplanned/unbudgeted” border deployment and the funding transfers.

In Monday's letter, the senators referenced Neller's memo while noting that other military officials have said recently that troop deployment has not affected readiness. They also asked Shanahan to explain those "discrepancies."

"To the extent the southern border deployment is causing readiness concerns, they must be addressed immediately," the senators added, while also requesting that Shanahan provide "a staff-level briefing on this matter within seven days" and a written explanation detailing how he will address Neller's concerns.

In addition to Warren and Gillibrand, Monday's letter was also signed by Sens. Jack Reed John (Jack) Francis ReedWhen 'Buy American' and common sense collide Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election Top Democrats press Trump to sanction Russian individuals over 2020 election interference efforts MORE (D-R.I.), Jeanne Shaheen Cynthia (Jeanne) Jeanne ShaheenSenate Democrats introduce bill to sanction Russians over Taliban bounties Trump-backed candidate wins NH GOP Senate primary to take on Shaheen Democratic senator urges Trump to respond to Russian aggression MORE (D-N.H.), Richard Blumenthal (D-Conn.) and Tammy Duckworth Ladda (Tammy) Tammy DuckworthMcConnell focuses on confirming judicial nominees with COVID-19 talks stalled Biden courts veterans amid fallout from Trump military controversies John Fogerty: 'Confounding' that Trump campaign played 'Fortunate Son' at rally MORE (D-Ill.).