The Republican Party has, for the most part, backed U.S. President Donald Trump through controversial policy announcements and Special Counsel Robert Mueller's investigation. But the prospect of supporting his proposed tariffs on all goods imported from Mexico seems a bridge too far.

Trump's threat of tariffs are a response to the migrant crisis at the southern border. The president wants Mexico to do more to curb the number of migrants at the border, or face more duties each month, beginning Monday at a level of five per cent.

"This is about immigration," said Kathleen Claussen, a former associate general counsel with the United States Trade Representative's office. "Some have said this is [Trump senior adviser for policy] Stephen Miller and others pushing this along."

But lawmakers warned the Trump administration they're considering ways to stop Trump's effort to impose the tariffs. One possibility, if they have the votes, is to assemble a two-thirds majority to disapprove the tariffs and also nullify a likely veto from the president.

They came up short of a veto-proof majority last February, when only a dozen Republicans voted to oppose Trump's national emergency declaration to reallocate $7 billion US in funds from other parts of the government to build his promised border wall. (That use of authority is being challenged in court.)

U.S. President Donald Trump said last week he would seek to apply five per cent tariffs on all goods imported from Mexico into the U.S., which Republican senators staunchly oppose. (Liam McBurney/Reuters)

This time, Republicans have cautioned a disapproval resolution could succeed. At least one Republican who backed Trump's first national emergency, Ron Johnson, told reporters: "This would be a different vote."

Republicans have expressed concern or disagreement with the president on Russia sanctions and his deference to Saudi Arabia following the slaying of journalist Jamal Khashoggi. On trade, they've been willing to break with him.

Mexican President Andrés Manuel López Obrador says Mexico will not respond to Trump's threat of coercive tariffs with desperation, but instead push for dialogue, during his daily news conference at the National Palace in Mexico City on May 31. (Ginnette Riquelme/Associated Press)

Veteran Kentucky Republican strategist Scott Jennings chalks it up not just to the party's penchant for viewing itself as the party of free trade, but also for the simple fact that many conservative lawmakers will invariably perceive tariffs as yet more taxes.

"If you're a Republican, you believe increased taxes means lower productivity," he said. "At some base level, Republicans are just nervous and immediately skeptical any time you're proposing tax increases."

Sen. Kevin Cramer, a Trump ally, told Politico: "A lot of Republican members of the Senate are tariff-weary. It's like, anything but tariffs."

Trump has said it would be "foolish" for Republicans to defy him.

Jennings sees the threat of a Senate disapproval resolution as a moot issue anyway. Veto-proof majorities would be needed in both the House and the Senate, "and it looks to me that in the House, it would be exceedingly difficult" to override Trump's veto, he said.

U.S. Trade Representative Robert Lighthizer was reportedly unhappy about Trump's proposal to impose tariffs on goods from Mexico. His office has backtracked, issuing a statement saying he supports the president. (Mandel Ngan/AFP/Getty Images)

Red states on edge

Meanwhile, lawmakers from red states are on edge about how slapping tariffs on Mexico will impact local economies. One Texas economic consulting firm, the Perryman Group, estimates 406,000 job losses nationally, 117,000 of which would come from the Lone Star State.

"Immigration and trade are entirely separate issues and need to be treated as such by our administration," Eddie Aldrete, chairman of the Texas-Mexico Trade Coalition said in a statement. "The U.S. does not need to be distancing themselves from Mexico but working with them in a united effort at the southern borders. The economic consequences of Trump's new plan could be swift and severe, especially in Texas."

Aside from U.S. firms having to pay import penalties and passing costs along to consumers, he said, the proposal would undermine Trump's effort to ratify the Canada-U.S.-Mexico Agreement.

Trade experts see a deep irony there, given that part of the reason for negotiating the new NAFTA in the first place was to get rid of tariffs.

That might explain reports that U.S. Trade Representative Robert Lighthizer was unhappy with Trump's move, though Lighthizer has since told the Wall Street Journal he "supports what the president is doing."

Texas Sen. Ted Cruz, right, has reportedly criticized the tariffs on Mexico as a $30-billion US tax on Texans. (Jonathan Ernst/Reuters)

'Not much support for tariffs'

The same cannot be said of Republican senators.

According to the New York Times, Sen. Ted Cruz called the proposed tariffs a $30-billion tax increase on Texans.

"There's no reason for Texas farmers and ranchers and manufacturers and small businesses to pay the price of massive new taxes," Cruz told reporters Tuesday.

Senate Majority Leader Mitch McConnell warned Trump's tariffs would be met with opposition from Republicans.

"There is not much support for tariffs in my [Senate Republican] conference, that's for sure," McConnell said.

Sen. John Cornyn attacked the wisdom of the tariffs as a self-harming move, saying it was akin to "holding a gun to our own heads," according to the Times.

Whether one agrees with them or not, former U.S. trade negotiator Ira Shapiro said Senate Republicans know the importance of trade to their constituents — especially in the lead-up to an election year in which economic rigour could help re-elect President Trump.

Sen. John Cornyn attacked the wisdom of the tariffs as a self-harming move, saying it was akin to 'holding a gun to our own heads,' according to the New York Times. (Aaron P. Bernstein/Reuters)

Interests clash

"They know what it does to their manufacturing, their farmers, their service people. They understand it," he said. "They think the politics of it are overwhelmingly in favour of more open trade and standing up for their farmers and for the people in industries that would be adversely affected."

Senators will have to weigh their voters' potential concerns about undocumented immigration flows against their livelihoods.

"This is a case where their interests in border security clashes directly with the economic interests of their state," Shapiro said.

If Trump is taking a gamble with tariffs against Mexico that might jeopardize a trade agreement and his political fortunes, he might at least be making progress. Top Mexican officials, including the foreign minister, met Wednesday at the White House with the vice-president to try to head off tariffs.

Trump wants the Mexican government to set up security at the southern border, target transnational criminal organizations and agree to a "Safe Third Country" agreement that would make Mexico a safe destination for people seeking asylum.

"We've been trying to get Mexico to the table on this [border] issue for 20 years," said Rachel Bovard, policy director of the Conservative Partnership Institute think-tank. "I don't think these tariffs need to be permanent if Mexico acts. They'll go away. But I think it's appropriate for the president to use the leverage he has."