With President Donald Trump on a state visit to the UK, Republican senators are locked in discussions over whether to block Trump's tariffs on Mexico, The Washington Post reported Monday.

Trump plans to impose 5% tariffs on Mexican goods next week and raise them by 5 percentage points monthly until October unless the country curbs migration to the US.

The GOP lawmakers reportedly see the tariffs as a tax hike on American consumers and businesses, and they may have a veto-proof congressional majority to face down the president.

Republicans have rebelled against Trump before. He was forced to issue the first veto of his presidency in March, when Republicans were among those who voted to block his declaration of a national emergency at the US-Mexico border.

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As President Donald Trump enjoys the pomp and splendor of his first state visit to the United Kingdom, Republican lawmakers back in Washington are reportedly plotting a dramatic act of defiance against him.

Sources familiar with their discussions told The Washington Post in an article published Monday that the party could move to block Trump's planned tariffs on Mexico in Congress.

The president announced in a tweet last week that unless Mexico acted to halt the flow of migrants into the US, he would slap 5% tariffs on Mexican goods, with the amount increasing by 5 percentage points a month until hitting 25% in October.

Republicans are said to be concerned that the tariffs would function as an extra tax on the US businesses and consumers who buy goods and services from Mexico. Senate Majority Whip John Thune told The Post that Congress was "going to want to probably be heard from" regarding the president's authority to impose the tariffs.

It would not be the first time congressional Republicans have defied the president. Twelve Republican lawmakers were among those who forced Trump to issue the first veto of his presidency in March after Congress voted to block his declaration of a national emergency at the US-Mexico border. The declaration allowed the president to divert millions of dollars in Pentagon funds to border security.

Republicans may again attempt to defy Trump's declaration of emergency, which also allowed him to impose tariffs on Mexico. Republican lawmakers reportedly believe that this time they may have the numbers for a veto-proof majority to override the national security declaration and block the president's Mexico tariffs conclusively.

Trade experts and lawmakers from both parties told Politico on Monday that the White House might even have to declare a new national emergency to impose the tariffs. The first declaration did not specifically authorize the president to impose tariffs, they believe.

Republicans are also warning that the planned tariffs could endanger Trump's renegotiated free-trade deal with Canada and Mexico. Sen. Joni Ernst of Iowa in a statement on Friday warned, "If the president goes through with this, I'm afraid progress to get this trade agreement across the finish line will be stifled."

Mexican officials on Monday warned that tariffs would do nothing to halt the number of people illegally crossing the border to the US and would damage the economies of both countries.

Mexico's economy minister, Graciela Márquez Colín, said the tariffs on agriculture could cause a combined $117 million a month in economic damage in the two countries.

Trump showed no sign of backing down. In a tweet on Monday he insisted it was in Mexico's power to act on his demands. "As a sign of good faith, Mexico should immediately stop the flow of people and drugs through their country and to our Southern Border," he said.

"They can do it if they want!"