President Trump’s reinvigorated effort to secure the Mexican border coincides with the acceleration of his 2020 reelection bid and comes as some immigration hawks say the administration has failed to realize a signature campaign promise.

Trump on Monday said the federal government would begin releasing undocumented immigrants in sanctuary cities, a provocative move to pressure Democrats in Congress to negotiate and that follows a bold house cleaning of top personnel at Homeland Security. The president wants the department to toughen its response to a historic surge of asylum-seekers and unlawful immigrants, vexing problems that persist despite his vow to halt illegal crossings.

“The president, I think, realizes that this is a real vulnerability, politically, for him,” said Mark Krikorian, who leads the Center for Immigration Studies, a group that favors reducing legal and illegal immigration. “They’re kind of panicking.”

Trump has prioritized immigration enforcement from Day 1, elevating the issue over the booming economy as he traversed the country campaigning for Republicans during the midterm elections. The strategy produced mixed results. Democrats picked up 40 seats and won control of the House of Representatives for the first time in eight years. Republicans held the Senate, gaining two seats even as they lost one each in Arizona and Nevada.

But as Trump’s own election nears, immigration hawks otherwise supportive of the administration’s aggressive border policies say the president could find himself exposed, politically, for lack of tangible results. Construction of new portions of wall along the southern border has been slow, and now Trump is dealing with a fresh crisis. Migrants from Central America are flooding into the U.S. from Mexico, dissuaded neither by the president’s tough talk nor border enforcement measures.

“We have this glaring problem on the southern border and [Trump is] in charge,” said Rosemary Jenks of NumbersUSA, a group that promotes reducing legal immigration. “There is just a huge frustration level for the president, because this is his signature issue. We’re going back into campaign mode.”

Trump allies confirm the president’s reenergized push to control the border and stem illegal immigration is a product of a frustration that has boiled over. Trump loyalists, like the president, blame a recalcitrant Congress and administration agencies that have passively resisted White House directives.

“There’s a growing frustration that there’s more that could be done, both legislatively, of which he doesn’t have control over, but more that could be done administratively that’s not being done,” Rep. Mark Meadows, R-N.C., told the Washington Examiner.

Democrats, in control of the House of Representatives since January, have held their ground on immigration, even during a record-long government shutdown Trump initiated to try, unsuccessfully, to force Congress to appropriate more money for construction of a border wall.

Now the president is focused on limiting asylum-seekers. Even many Republicans in Congress who do not share the president’s suspicion of immigration say existing statutes are being gamed by unqualified applicants.

Trump’s fresh strategy to force Congress’ hand appears to be a plan to funnel detained undocumented immigrants to sanctuary cities — local jurisdictions that have pledged to protect illegal residents from deportation.

These are generally Democratic strongholds, although some are near Republican-leaning communities. This is on the heels of Trump firing the top leadership at Homeland Security, the department that oversees immigration and border security, including now former Secretary Kristjen Nielsen.

“Those Illegal Immigrants who can no longer be legally held (Congress must fix the laws and loopholes) will be, subject to Homeland Security, given to Sanctuary Cities and States!” Trump tweeted.

Trump’s position on immigration, criticized as extreme and xenophobic by Democrats, and occasionally by some Republicans, was nevertheless a crucial asset in his victory over Democrat Hillary Clinton in 2016. He is among the few politicians of either party to commit to curbing illegal border crossings and deporting undocumented immigrants. The GOP base remains supportive.

But overall response to the president’s leadership on immigration since entering the White House has been mixed. Indeed, Trump’s approval rating on immigration was a paltry 41% in the Georgetown University Battleground Poll.

By contrast, Trump enjoyed a 58% approval rating on the economy in the same survey, jointly conducted March 31 to April 4 by Democratic and Republican pollsters. The Trump campaign shrugged off the criticism, saying the president is getting things done at the border.

"There has been no greater leader on border security than President Trump. While past presidents have failed to act, President Trump has shown the political courage to tackle an issue long ignored by Washington,” spokeswoman Kayleigh McEnany said.