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"Your decision endangers our national security and signals to the Kremlin that the United States is not willing to stand up to its aggression," the senators wrote

They added that "instead of sending a signal that could be interpreted by Vladimir Putin as an invitation to further aggression in Europe" that Trump should "support U.S. national security interests and reverse this decision."

The Pentagon announced last month that it would move roughly $770 million in European Deterrence Initiative funding toward the border wall as part of the larger $3.6 billion being shifted as part of Trump's emergency declaration.

The program, started after Russia annexed Crimea from Ukraine, is meant to support U.S. troops in Europe, including training with allies.

The Democratic senators argued that the cuts raised questions about Trump's commitment to NATO, an international organization that the president has repeatedly criticized.

"These cuts signal to the Kremlin that you do not view its interference in Europe as a serious concern and potentially serve as a green light for Moscow to expand their malign activities," the senators added in their letter.

Lawmakers are currently debating whether or not to backfill the $3.6 billion as part of their fiscal 2020 spending bills.

House Democrats didn't authorize or appropriate the money as part of a mammoth defense policy bill, known as the National Defense Authorization Act (NDAA), or as part of their military construction–veterans affairs spending bill.

The Senate included language replenishing the funding in the NDAA and is expected to include similar language in their military construction–veterans affairs spending bill, if it is released.