The White House is considering delaying tariffs against Mexico as the two sides continue to negotiate, but U.S. President Donald Trump defended the levies, calling them “a beautiful thing.”

“I have these people — and I’m saying there are some Republicans, too — I think they should be ashamed of themselves … they come out and they talk about ‘tariffs are bad.’ So they are killing — they are hurting my negotiation.”

The two sides have been meeting in Washington D.C. this week to try and come to an agreement, and Mexico is pushing for more time to figure out exactly what steps it needs to take in order to avoid the tariffs. Trump has said they would go into effect on Monday, June 10, starting at 5% and rising an additional 5% each month, up to 25%, until Mexico stops the flow of illegal immigrants.

Republican senators have threatened to block Trump’s tariffs, which are effectively a tax on American citizens, who will pay more for a wide range of goods and foods from avocados to tomatoes to beer. Others, however, say the tariffs are a necessary evil in order to gain leverage in the negotiations.

The problem for Trump is there are a number of senators from his own party who don’t believe he knows “how to use them properly,” and think he could end up sending the economy into recession, in addition to hitting U.S. consumers and companies in their wallets. Trump has incorrectly said a number of times that Americans aren’t paying the tariffs, and that the U.S. is bringing in billions of dollars from them.

“There’s a point where tariff fatigue sets in,” said Sen. Mike Braun, R-Ind., a close Trump ally. “I don’t know what might occur and I’ve really not heard anything whether there’s been any progress.”

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

Sen. Rand Paul, R-Ky., said he believes there is a veto-proof majority of senators opposed.

“Why suffer a losing vote that maybe they’d put up here in the Senate?” said Paul, who is close to Trump. “I think (new tariffs are) a bad idea and I think there’s a possibility that there could be a veto proof-majority.”

Trump took issue with Republicans fighting the tariffs.

Congress “should should be saying, ‘We are with the President, we will do whatever he wants to do,’ and Mexico would fold like an umbrella,” he said. “I have these people — and I’m saying there are some Republicans, too — I think they should be ashamed of themselves … they come out and they talk about ‘tariffs are bad.’ So they are killing — they are hurting my negotiation.”

At issue for Mexico is exactly what it needs to do to avoid the tariffs, and whether it has the capabilities.

Secretary of Homeland Security Kevin McAleenan says there are three things Mexico needs to do: Secure the border between Guatemala and the Mexican state of Chiapas, crack down on organizations that help migrants travel through Mexico, and then align with the U.S. on asylum, stipulating that Mexico is a safe place for people fleeing Central America to stay.

According to a Reuters report, Mexican soldiers, armed police and migration officials blocked a caravan of hundreds of migrants after they crossed the border from Guatemala into southern Mexico on Wednesday, detaining dozens of them.

Mexico’s National Migration Institute (INM) said via statement that a group of about 300 people entered into Mexico by crossing a border bridge, and another 120 people joined the group walking into the city of Tapachula. The migrants reportedly agreed to travel to a migration office for processing.

During the month of May alone, border patrol apprehended more than 132,000 people crossing from Mexico, a third more than April and the highest monthly number since 2006, reaching “crisis” levels, according to U.S. officials.

Mexico has reportedly created a militarized police force called the National Guard, made up of soldiers and federal police. Mexican police also detained the director of the U.S.-Mexico migrant aid group, Irineo Mujica, and Cristobal Sanchez, a migrant rights activist.

Mujica’s group, Pueblo Sin Fronteras, has guided migrant caravans through Mexico in an attempt to protect them from corrupt officials, human traffickers and extortionists.