“This is really an evil policy. And no matter how Trump tries to change the topic to something else, it is not OK to hurt children to send a message,” he said in an interview. “And it is not OK to hurt children in order to gain what Trump said was legislative leverage.”

If Trump thought he could force Democrats to accept more money for a border wall or more draconian immigration concessions by enforcing a “zero tolerance” enforcement policy, it was clear by Monday that he had miscalculated. The situation has the makings of another travel ban moment, as public outrage rises in an echo of the airport protests in the winter of 2017 stemming from Trump’s restrictions on travel from majority-Muslim nations.

The separation of migrant families is drawing a growing number of GOP critics, from Michigan Rep. Fred Upton to Massachusetts Gov. Charlie Baker, and Democrats are unified and sensing an advantage.

The day after Merkley’s trip through the sweltering Rio Grande Valley, another half-dozen Democrats visited a Texas shelter for migrant children while House Minority Leader Nancy Pelosi (D-Calif.) made her own trip to the California border. As legislation to prevent family separations won backing from Capitol Hill’s most prominent Trump-friendly Democrat, West Virginia Sen. Joe Manchin, the party was finally speaking in one voice on immigration ahead of November’s midterms.

The newfound Democratic unity on immigration is particularly notable given the still-bitter aftertaste of a government shutdown that the party courted this winter in a bid to win relief for Dreamers, undocumented immigrants brought to the United States as children. Liberals fumed as Senate Democratic leaders gave in on the shutdown, while moderates lamented the entire strategy as misguided.

Yet all those recriminations appeared far in Democrats’ rearview mirror by Monday. Their base was roaring in fury over separated migrant families as a new Quinnipiac poll showed that 66 percent of voters oppose the administration’s policy.

Whether Democrats can coax worried Republicans into forcing the administration to stop remains to be seen, since GOP leaders have rallied around an approach that advocates warn would increase detentions while not necessarily keeping more families together. Either way, Trump has succeeded in galvanizing Democratic lawmakers and voters across the party’s ideological spectrum.

“I’m proud to see Democrats united,” Rep. Peter Welch (D-Vt.) said during a stop on the Merkley-led tour, “and I’m proud to see some of my Republican colleagues” supporting a House floor debate on immigration beyond the immediate outcry over separated families.

Republicans have responded to the uproar by pointing out that the Obama administration stepped up deportations and reminding the public that the consequences that migrants face for illegal entry are no different than for those faced by an American who commits a crime. Trump aides also are planning a fresh tightening of immigration rules ahead of November, indicating that Democrats aren’t alone in using the flap to play to their base.

“Obviously, if an American citizen has a child and they’ve committed a crime, they’re separated right away as well,” Sen. James Lankford (R-Okla.) said in an interview last week.

Senate Majority Whip John Cornyn (R-Texas) said Monday that Congress should expedite hearings for people apprehended at the border and change the law to stop separations.

“We want to try and deal with this problem [in] as humane and compassionate a way as we can,” he said. “We also need to enforce our borders and enforce our law. I don’t think those are inconsistent.”

Indeed, Homeland Security Secretary Kirstjen Nielsen has defended separations as a necessary consequence of prosecuting migrants who have committed the first-time misdemeanor crime of illegal entry. The technical reality of the policy, however, is worlds away from the political message it sends, which Republicans are well aware of.

Lankford, a former minister who publicly blanched at the White House’s invocation of the Bible to justify its policy, made clear that he’d “prefer to have families together as much as you possibly can.” Sen. Ben Sasse (R-Neb.), who urged the administration on Monday to end family separations, said in a statement that he is working with Lankford on a solution.

The Department of Health and Human Services-run system that shelters migrant children has become a political live wire as its numbers grow, with the administration reporting Monday that it currently holds 11,785 unaccompanied minors who illegally entered the U.S., alone or with parents.

Reporters were not permitted to accompany the Democratic lawmakers on their Sunday or Monday tours of the South Texas facility run by a nonprofit contractor, Southwest Key.

Despite the growing Republican skittishness, Democrats have yet to attract a GOP cosponsor for legislation to halt the family separation policy. A Democratic counterpart bill in the House could put further pressure on Republicans should it attract backing from moderate critics of the administration’s decision such as Upton and Rep. Will Hurd (R-Texas).

In the meantime, Welch said, he’s hearing from constituents who don’t associate their opposition to the family separation policy with Trump or even political partisanship.

“What’s really motivating them,” he said, “is their horror at the suffering of these kids.“

Burgess Everett contributed to this report from Washington.