WASHINGTON — The Trump administration is planning to dispatch at least 800 active-duty troops to the southern border as a caravan of thousands of migrants makes its way through Mexico.

Defense Secretary Jim Mattis is expected to sign an order sending the troops to the border, bolstering National Guard forces already there, an official said Thursday. The action comes as President Donald Trump has spent recent days calling attention to the caravan of Central Americans slowly making its way by foot into southern Mexico but still more than 1,000 miles from U.S. soil.

Trump, who made fear about immigrants a major theme of his 2016 election campaign, has been eager to make it a top issue heading into the Nov. 6 midterm elections, which will determine control of Congress. The president and senior White House officials have long believed the issue is key to turning out his base of supporters.

The additional troops would provide logistical and other support to the Border Patrol, said aU.S. official, who spoke on condition of anonymity to discuss a plan that had not been finalized and formally announced.

It’s not unusual for the National Guard to help with border security. Active-duty troops, however, are rarely deployed within the U.S. except for emergencies like hurricanes or floods. Fears of militarizing the border were fanned by a May 1997 incident in which a Marine on a counter-narcotics mission shot to death an 18-year-old who was herding goats in Redford, Texas.