A group of likely Democratic presidential candidates on Tuesday challenged the Trump administration over its deployment of 5,800 troops to the U.S.-Mexico border shortly before the midterm elections.

In a letter to Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE, a group of 2020 Democratic hopefuls, including Sens. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.), Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (D-N.J.) and Kamala Harris Kamala HarrisHarris faces pivotal moment with Supreme Court battle Nearly 40 Democratic senators call for climate change questions in debates Joe Biden has long forgotten North Carolina: Today's visit is too late MORE (D-Calif.), wrote they were “deeply disturbed by the overt politicization of the military.”

They argued the deployment of troops along the border to intercept a caravan of migrants from Central America was not justified by national security concerns. They charged that it was really designed to bolster President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE’s campaign message on illegal immigration.

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“The politically-motivated mission misuses active duty servicemembers and wastes Department of Defense (DOD) resources, both of which could compromise military readiness in the face of a true national security threat and heighten the risk of a confrontation between asylum-seekers and armed servicemembers,” they wrote.

The lawmakers called for a briefing and written justification from the U.S. Northern Command for the deployment of troops along the border and urged Mattis to otherwise “curb the unprecedented escalation of DOD involvement in immigration enforcement.”

The senators criticized Northern Command for issuing what they called an “inflammatory” order days before the elections that “exaggerated the migrant threat.

The order warned the security of the nation was “imperiled by a drastic surge of illegal drugs [and] dangerous gang activity.”

The Democrats say the late-October order contradicted another intelligence assessment that predicted the number of migrants in the caravan was likely to decrease and found the group was not likely infiltrated by terrorists or criminal gangs.

Senate Democrats say that active-duty troops didn’t need to be deployed to the border because National Guard units were already stationed in the area. They argued that the combined number of U.S. troops on the border was greater than the number of migrants expected to arrive.

“Our concerns about this seemingly disproportionate military response to the migrant caravan are amplified by the obvious political agenda at play. Both the deployment timing and mission name reveal a blatant partisan ploy,” they wrote.

Trump crisscrossed the country in the week before Election Day warning of the threat posed by the caravan as he bashed Democrats for being weak on border security.

The senators urged Mattis “to protect the apolitical, non-partisan nature of the U.S. Armed Forces” and gave him a deadline of Dec. 6 to provide a briefing justifying the deployment.

The other signatories to Tuesday’s letter include Sens. 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.), Sherrod Brown Sherrod Campbell BrownHarris faces pivotal moment with Supreme Court battle Remote work poses state tax challenges Senate Democrats release report alleging Trump admin undermined fair housing policies MORE (D-Ohio) and Jeff Merkley Jeffrey (Jeff) Alan MerkleyThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Sunday shows - Trump team defends coronavirus response Oregon senator says Trump's blame on 'forest management' for wildfires is 'just a big and devastating lie' MORE (D-Ore.), who are also considering presidential bids in 2020.

Sens. Bob Menendez Robert (Bob) MenendezKasie Hunt to host lead-in show for MSNBC's 'Morning Joe' Senators ask for removal of tariffs on EU food, wine, spirits: report VOA visa decision could hobble Venezuela coverage MORE (D-N.J.), Ben Cardin Benjamin (Ben) Louis CardinCongress must finish work on popular conservation bill before time runs out PPP application window closes after coronavirus talks deadlock Congress eyes tighter restrictions on next round of small business help MORE (D-Md.) and Ed Markey Edward (Ed) John MarkeyDemocrats shoot down talk of expanding Supreme Court Schumer: 'Nothing is off the table' if GOP moves forward with Ginsburg replacement Democrats see fundraising spike following Ginsburg death MORE (D-Mass.), members of the Senate Foreign Relations Committee, and Sens. Ron Wyden Ronald (Ron) Lee WydenDemocrats call for declassifying election threats after briefing by Trump officials Read Democrats' report countering Republicans' Biden investigation Top GOP senators say Hunter Biden's work 'cast a shadow' over Obama Ukraine policy MORE (D-Ore.) and Chris Van Hollen Christopher (Chris) Van HollenCongress must finish work on popular conservation bill before time runs out Democrats fear Russia interference could spoil bid to retake Senate Mid-Atlantic states sue EPA over Chesapeake Bay pollution MORE (D-Md.) also signed the letter.

Booker and Merkley are also member of the Foreign Relations panel.