Trump frequently spends his mornings live-tweeting Fox & Friends, and his tweet almost certainly came in response to the show's coverage of the story that day:

This is the same segment that covered the Washington Examiner op-ed Trump tweeted about a few minutes ago.

With Trump weighing in, the story’s coverage escalated. And on Wednesday night, Fox contributor and presidential confidant Newt Gingrich urged Republicans to make the caravan a key voting issue:

Newt Gingrich: Two words will define the 2018 election – Kavanaugh and caravan (via #Hannity) pic.twitter.com/MNMRDPeBAi — FOX & friends (@foxandfriends) October 18, 2018

“I think two words are going to define the night of the 2018 election in the next three weeks,” he told Sean Hannity. “One is Kavanaugh and the other is caravan.” Claiming that “the left is eager” for the caravan to enter the United States, Gingrich argued that “the American people are going to reject ... the way they are dealing with the border, and I think those will end up being the reasons the Republicans keep the House and dramatically increase the number of senators they have.”

The next morning, Fox & Friends repeatedly urged Republicans to take Gingrich’s advice.

During the show’s lead segment, after several minutes of dire warnings about the caravan, the hosts replayed portions of Gingrich’s comment. “So it comes down to a simple question regarding the Republicans and the Democrats, because it’s clear,” said host Steve Doocy. "If you think that our southern border should be open, support the Democrat. If you think the southern border should actually be a border with security, and stopping people, and processing them accordingly, then you’ve got to vote for Republicans, the Republicans say.”

In a second segment that hour, Fox contributor and former ICE Acting Director Tom Homan said, “This caravan issue lays at the feet of the Democratic Party up on the Hill” for not closing loopholes in immigration law. “I hope the American people are paying attention because this isn’t the president’s failure, this isn’t the secretary’s failure; this is the Democrats’ failure because they know the issue and they refuse to fix it. They’re putting their political ambitions ahead of public safety, national security, and border control.”

During a third segment, Doocy said that the election is “going to come down to” voters asking each other, “Hey, did you see that story this morning on Fox & Friends about the caravan? Can you believe that the Democrats want open borders?”

And at 7 a.m., the hosts again highlighted the migrant caravan, with Doocy arguing that “these images do get the base on the Republican side interested in voting because, clearly, it's a choice. Do you want a southern border with security or not?”

The president apparently got the message. In a series of tweets beginning at 7:25 a.m., he used the caravan to attack Democrats, saying they had “led (because they want Open Borders and existing weak laws)” an “assault on our country.” He also threatened to stop "all payments” to Honduras, El Salvador, and Guatemala, and said he would “call up the U.S. Military and CLOSE OUR SOUTHERN BORDER” if Mexico could not “stop this onslaught.”

I am watching the Democrat Party led (because they want Open Borders and existing weak laws) assault on our country by Guatemala, Honduras and El Salvador, whose leaders are doing little to stop this large flow of people, INCLUDING MANY CRIMINALS, from entering Mexico to U.S..... — Donald J. Trump (@realDonaldTrump) October 18, 2018

....In addition to stopping all payments to these countries, which seem to have almost no control over their population, I must, in the strongest of terms, ask Mexico to stop this onslaught - and if unable to do so I will call up the U.S. Military and CLOSE OUR SOUTHERN BORDER!.. — Donald J. Trump (@realDonaldTrump) October 18, 2018

....The assault on our country at our Southern Border, including the Criminal elements and DRUGS pouring in, is far more important to me, as President, than Trade or the USMCA. Hopefully Mexico will stop this onslaught at their Northern Border. All Democrats fault for weak laws! — Donald J. Trump (@realDonaldTrump) October 18, 2018