In a speech at the White House designed to keep the focus on immigration ahead of the midterm elections, Trump ramped up his attacks on a migrant caravan from Central America that is heading toward the U.S., calling it an "invasion" that poses a threat to the nation.

"They want to throw rocks at our military, our military fights back," he said. "I told them, consider it a rifle."

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Trump portrayed the caravan as a national "crisis," though the group is thousands of miles from the southern border and is said to mostly comprise people fleeing poverty and violence. The president claimed the caravan is full of "tough people" who had "violently overrun the Mexican border" and "hurt badly Mexican soldiers."

Trump has repeatedly returned to the threat of immigration in the closing week of the midterm election campaign, seeking to stoke fears among his base about immigration to get them to the polls. There are signs the strategy has helped a number of Senate Republican candidates, though it has clearly made GOP members of the House uncomfortable.

The GOP is expected to keep or build on its Senate majority next week, but is favored to lose its House majority.

Critics called the claim of an invasion another political stunt by Trump and pointed out that the 5,200 active-duty troops the president deployed to the border are serving in a support role and not tasked with apprehending or confronting migrants.

U.S. law generally prohibits the military from participating in law-enforcement activities on American soil. That could change in the event of a national emergency, but Trump has not declared one regarding the caravan.

"Our men and women in uniform are better trained, better equipped, and better led so they meet any threat with confidence. A wasteful deployment of over-stretched Soldiers and Marines would be made much worse if they use force disproportional to the threat they face. They won’t," tweeted retired Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff under former President Obama.