With less than two weeks before the midterm elections, the Pentagon is preparing to deploy at least 800 troops to the U.S.-Mexico border to confront a migrant caravan that President Donald Trump has described as a “national emergency,” said administration officials.

Defense Secretary Jim Mattis may sign an order mobilizing those troops as early as Thursday, according to two administration officials, who spoke on condition of anonymity. They said the number of troops will range from 800 to 1,000.

The news of Mattis' decision follows a Thursday morning tweet from Trump who vowed to halt the Central American caravan by "bringing out the military."

The troops would add to the roughly 2,100 National Guard soldiers Trump already spread out across the border under an order from Trump earlier this year. The troops are not engaging with migrants directly or performing law enforcement duties. Instead, they’re backing up Customs and Border Protection by monitoring video of the border and performing other tasks to free up Border Patrol agents to do their job in securing the border.

It remains unclear whether the latest deployment would call up more National Guardsmen, or whether the Pentagon is considering sending active-duty troops to the border, an idea floated by Trump that would face many legal hurdles and congressional scrutiny.

Even if Trump avoids that controversy by mobilizing more National Guard troops, critics say the move is a "desperate political stunt" designed to stoke anti-immigrant fears in the leadup to the Nov. 6 elections.

"The caravan is in no way a threat and Sec. Mattis should be refusing to use U.S. military personnel as political props in Trump's war on immigrants," said Heidi Hess, co-director of CREDO Action, a liberal network that advocates for social change.

Members of Congress have also been skeptical of Trump's reaction to the migrant caravan, which is slowly making its way through southern Mexico more than 1,000 miles away from the U.S. border.

Asked about deploying troops to the border, Sen. Ben Cardin, D-Md., told CNN this week that the president "should be looking for ways of providing the needs for these individuals before they hit our borders."

"There should be a way that they can present (asylum) claims and know that they'll be presented fairly when they reach the border," he said.

Meanwhile, Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa, and Sen. Mike Lee, R-Utah, want the Trump Administration to seek a "safe third country" asylum agreement with Mexico to deal with the caravan migrants. Such an agreement would require asylum-seeking migrants to make their claim in the first country of arrival rather than passing through to another country, according to Grassley and Lee.

"Entering into a safe third country agreement with Mexico would send a message to our partners across Central America that they too must share the burden of unsanctioned mass migration," said the senators in a joint statement.

Mexican President Enrique Peña Nieto has already asked the United Nations to help his government process the thousands of migrants who are trying to claim asylum.

While Trump has warned that members of the migrant caravan are trying to sneak into the country, most migrants who participated in a separate caravan earlier this year presented themselves at ports of entry to request asylum.