President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE on Monday doubled down on his call to drastically ramp up border security and overhaul the immigration system a day after he threatened to shut down the government over the issue.

"We must have Border Security, get rid of Chain, Lottery, Catch & Release Sanctuary Cities - go to Merit based Immigration," he tweeted. "Protect ICE and Law Enforcement and, of course, keep building, but much faster, THE WALL!"

We must have Border Security, get rid of Chain, Lottery, Catch & Release Sanctuary Cities - go to Merit based Immigration. Protect ICE and Law Enforcement and, of course, keep building, but much faster, THE WALL! — Donald J. Trump (@realDonaldTrump) July 30, 2018

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Trump's tweet came a day after he said on Twitter that he would be willing to "shut down" the government if Democratic lawmakers did not agree to his demands on immigration and border security, including the construction of his long-promised wall along the U.S.-Mexico border.

Trump has repeatedly called for an end to family-based immigration, which he refers to as "chain migration," as well as the diversity visa lottery program.

Lawmakers face a spending deadline in September. If Trump were to allow the government to shut down, it would mark the second lapse in funding this year. The first happened in January amid a dispute over protections for beneficiaries of the Deferred Action for Childhood Arrivals (DACA) program, which Trump rescinded last year.

More recently, the president has come under intense criticism for his zero-tolerance immigration policy, which prioritizes prosecutions of people who enter the U.S. illegally.

That policy led to thousands of children being separated from their parents at the southern border. A federal judge ordered the government last month to swiftly reunite those families.