President Trump declared Wednesday evening that "not nearly enough" progress was being made in last-minute negotiations with Mexico, as the U.S. prepares to impose escalating tariffs unless that nation does more to stop the rush of illegal immigrants arriving at the southern border.

"Immigration discussions at the White House with representatives of Mexico have ended for the day," Trump tweeted. "Progress is being made, but not nearly enough!"

Trump continued: "Border arrests for May are at 133,000 because of Mexico & the Democrats in Congress refusing to budge on immigration reform. Further talks with Mexico will resume tomorrow with the understanding that, if no agreement is reached, Tariffs at the 5% level will begin on Monday, with monthly increases as per schedule. The higher the Tariffs go, the higher the number of companies that will move back to the USA!"

Mexico Foreign Minister Marcelo Ebrard told reporters that the negotiations mostly concerned migration, and that tariffs would be the focus on Thursday. Earlier this week, Ebrard was optimistic, saying "we will be able to reach an agreement."

In a statement last week, the White House set out a punishing schedule for planned tariff hikes.

'FULL-BLOWN EMERGENCY' DECLARED AS ARRESTS SKYROCKET AT BORDER IN MAY

"If Mexico still has not taken action to dramatically reduce or eliminate the number of illegal aliens crossing its territory into the United States, Tariffs will be increased to 15 percent on August 1, 2019, to 20 percent on September 1, 2019, and to 25 percent on October 1, 2019," Trump said in a statement released last Thursday. "Tariffs will permanently remain at the 25 percent level unless and until Mexico substantially stops the illegal inflow of aliens coming through its territory."

The statement added: "Thousands of innocent lives are taken every year as a result of this lawless chaos. It must end NOW! ... Mexico’s passive cooperation in allowing this mass incursion constitutes an emergency and extraordinary threat to the national security and economy of the United States."

The number of migrants apprehended at America's southern border skyrocketed last month to levels not seen in over a decade, with U.S. Customs and Border Protection reporting nearly 133,000 arrests in May.

The number surpassed 144,000 when counting migrants deemed inadmissible -- a more than a 30 percent increase from the prior month and double the influx recorded at the beginning of the year.

"We are in full-blown emergency," a CBP official said Wednesday. The number of apprehensions was the highest monthly total in more than 13 years. In April, authorities recorded 99,304 arrests.

However, congressional Republicans are weighing a resolution of disapproval against Trump's proposed tariffs. A two-thirds majority of both the Democrat-controlled House and the GOP-controlled Senate would be needed to kill the tariffs.

Senate Majority Leader Mitch McConnell said with understatement, “There is not much support in my conference for tariffs, that’s for sure.”

“Deep concern and resistance,” is how Sen. Ted Cruz of Texas characterized the mood. “I will yield to nobody in passion and seriousness and commitment to securing the border, but there’s no reason for Texas farmers and ranchers and manufacturers and small businesses to pay the price of massive new taxes.”

But other Republicans were more supportive. Sen. David Perdue, R-Ga., told Fox News that Trump needs to use a "hammer" because "we're being invaded right now."

Perdue could face a competitive re-election bid next year in a state which now has some Democratic leanings.

South Dakota Republican Sen. John Thune told Fox News he didn't know where most Republicans in the Senate stood on the issue.

"It’s hard to know. We haven’t whipped it," Thune said. 'This is all hypothetical."

Lawmakers and business allies worry the tariffs will derail the long-promised United-States-Mexico-Canada trade deal— a rewrite of the North American Free Trade Agreement that Trump campaigned against and promised to replace.

Sen. Chuck Grassley, R-Iowa, chairman of the Finance Committee, told reporters Tuesday the tariffs make passage of USMCA “more difficult.”

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But speaking to Fox News last week, senior officials said they anticipated no issues with the USMCA.

Fox News' Brooke Singman, Patrick Ward, and The Associated Press contributed to this report.