WASHINGTON — Shortly before President Donald Trump arrived for a campaign rally in central Wisconsin earlier this week, House Speaker Paul Ryan, R-Wis., took the podium. Before Ryan could start his stump speech, the crowd of fellow Wisconsinites interrupted him with a chant of "build the wall."

Trump's base is frustrated with Ryan and other Republican congressional leaders for not having supplied the $25 billion the president is seeking for a border barrier between the U.S. and Mexico. A caravan of mostly Honduran migrants making its way north has only put a fine point on that anger in the final days before the midterm elections.

That may help explain why Trump is making high-profile moves to show that he intends to prevent the caravan from getting to — or crossing — the U.S. border. The president is considering a proposal to make it much more difficult for Central Americans to receive asylum, and he is sending 800 troops to the border.

It is both his natural inclination and a response to the demands of his political base, said a former Trump White House official who spoke to NBC News on the condition of anonymity.

"Remember, this is not him just randomly saying out of the blue, 'I’m going to send troops to the border because we don’t have the wall yet.' It’s him responding to this caravan," the former official said. "If he didn’t respond, it would have deflated" the base.

Trump has been accused of politicizing the caravan, and of stoking fear, to help Republicans before next month's elections. The possibility of more troops heading to the border has raised the question of whether the military presence is an omen that more extreme force will be used against migrants.

His Homeland Security secretary, Kirstjen Nielsen, appeared to leave fodder for that interpretation in a Thursday interview with Fox News in which she said, "we do not have any intention right now to shoot at people."

A senior Department of Homeland Security official, speaking on the condition of anonymity because he was not authorized to discuss the topic publicly, told NBC News that there's no foreseeable circumstance — short of the fear of imminent harm, like a shootout situation — in which U.S. troops would be authorized to shoot at migrants.

While they are on the border, troops will act as part of agency-led teams, assist in detention and operate under the engagement protocols of civilian law enforcement authorities, the official said.

The value of having U.S. troops at the border is the deployment of additional sensor technology and to act as a force multiplier for civilian authorities, he said.

But the president has done little to clarify that U.S. troops will essentially be performing civilian functions. Instead, he has vaguely asserted that American forces are there to prevent border crossings — without outlining the limits of their power.

"I am bringing out the military for this National Emergency," he tweeted Thursday. "They will be stopped!"

Brandon Judd of the National Border Patrol Council is right when he says on @foxandfriends that the Democrat inspired laws make it tough for us to stop people at the Border. MUST BE CHANDED, but I am bringing out the military for this National Emergency. They will be stopped! — Donald J. Trump (@realDonaldTrump) October 25, 2018

Rep. Steve King, R-Iowa, a leading immigration hard-liner who is close to the president, pointed to the political necessity of Trump using the military as a deterrent at the border last week, according to Radio Iowa.

"It’s one of those promises that has to be kept,” King said. "I think about when Ronald Reagan said to the air traffic controllers: 'You have a chance to come back to work, but if you don’t, I’m going to fire you.’ And some said, ‘He doesn’t have the nerve to do that,’ but Ronald Reagan had to do that if he wanted to maintain his presidency, and this is one of those kind of circumstances, too.”

But in an area where politics and policy intersect, Jenny Beth Martin, co-founder of the Tea Party Patriots, said it is much more about the latter than the former among the Trump supporters with whom she speaks.

"Couldn’t that be the biggest driver of it right now, that there are thousands of people heading right now to our southern border?" she asked rhetorically.

She said Trump's base is already energized to vote because of the hot-button hearings on Supreme Court Justice Brett Kavanaugh's confirmation.

"I think the base has been riled up since the Kavanaugh hearings," she said. "I hear people asking, 'What’s going to happen, what are we going to do about this caravan?' but I don’t hear people saying, 'This is going to make me vote.'"

And yet the former White House official acknowledged the political imperative: Trump, said this former aide, is doing "what his supporters want and expect him to do on the border."