President Donald Trump has weaponized social media in his war on immigrants. This week, U.S. Sen. Jeff Merkley (D-Ore.) fired back at the White House.

When he live-streamed a failed visit to a detention center for immigrant children, Merkley set off a chain of increasingly horrified reactions to President Trump's "zero tolerance" policy that separates children from their parents when families illegally cross the U.S. border.

The federal contractors running the detention center in a former Walmart in Brownsville, Texas, called the police when Merkley requested access to the center at the facility's front door. The Facebook video of the encounter went viral, and a cascade of events has led to a crisis for the White House, which now faces widespread condemnation of its policy.

Merkley, who is widely rumored to be preparing an outsider bid for the 2020 Democratic nomination for president, wasn't the first person asking questions about the policy or the detention center. But calling the cops on Merkley sparked new curiosity from national media. And Merkley has become perhaps the leading Democratic voice demanding the policy's end.

Here's how Merkley took revelations about Trump's policy and used his newly elevated profile to push the issue.

June 3

What happened: A Texas newspaper, The Brownsville Herald, first reported in February 2017 that an abandoned Walmart Super Center would be converted into a detention center for immigrant children.

What Merkley did: Merkley shot a Facebook Live video June 3 as he was turned away from the detention center. The police showed up—giving Merkley national media attention. "Americans should be outraged by the fact that our taxpayer dollars are being used to inflict spiteful and traumatizing policies on innocent children," he said.

June 7

What happened: The White House made a mistake—it engaged. "No one is taking a public safety lecture from Sen. Merkley," a spokesman said, "whose own policies endanger children, empower human smugglers and drug cartels, and allow violent criminal aliens to flood into American communities."

What Merkley did: He demanded details. He asked the Office of Refugee Resettlement how many children had been separated from their parents at the border, what their ages were, and where they were being held. He set a June 15 deadline. (The federal office missed the deadline to respond, but new numbers have surfaced.) "The White House is attacking me because their cruel policy of stealing children from parents has no defense," he tweeted.

June 13

What happened: Reporters toured the detention center in Brownsville. MSNBC reporter Jacob Soboroff tweeted that journalists "likely wouldn't have had a shot at getting inside Casa Padre tonight had @SenJeffMerkley not tried and been turned away before us."

What Merkley did: The media tour sparked massive outrage—especially the revelation that children were being held in cages. And Merkley used that outrage to vindicate his criticism. "Now I'm headed back to the border," he tweeted, "and this time I'm bringing friends."

June 16

What happened: Oregon Public Broadcasting reported 123 men had been transferred to the Federal Correctional Institution in Sheridan, Ore.—some of them separated from their children.

What Merkley did: He joined fellow Sen. Ron Wyden and Reps. Suzanne Bonamici and Earl Blumenauer in Sheridan, touring the prison and holding a press conference. His new profile meant his quote led the next morning's front-page story in the Sunday New York Times. "This is not a zero tolerance policy, this is a zero humanity policy, and we can't let it go on," Merkley said.

June 17

What happened: Merkley returned to the detention center in Brownsville, joined by six other congressmen. This time, officials allowed the lawmakers to tour the facility and see the children detained there.

What Merkley did: He live-streamed this trip, too. This time, his visit—on Father's Day—led national news broadcasts and landed on front pages.

June 18

What happened: By Monday, all 49 Senate Democrats had signed on to the Keeping Families Together Act that would end family separations at the border. ProPublica released a tape of children wailing while a federal agent joked, and the White House faced international revulsion while defending itself with semantics.

What Merkley did: He demanded the resignation of U.S. Homeland Security Secretary Kirstjen Nielsen. "The WORD cages isn't the problem," he tweeted. "The problem is that kids are being ripped from their parents and put in those cages to send a message to others fleeing persecution. Horrific!"