U.S. Sen. Corry Gardner, a Republican, will oppose imposing a 20-percent import tax on Mexican imports, a sign that the Trump administration may face difficulties convincing Congress to back the proposal.

Gardner, who was recently tapped to head Senate Republicans’ campaign fundraising with the National Republican Senatorial Committee, said he fears the tax would start a trade fight that would cripple critical agricultural and other Colorado exports.

“A tariff war is a wrong thing to proceed,” Gardner told an audience Friday at the Colorado Water Congress annual convention at a hotel in the Denver Tech Center.

“I don’t support a 20-percent tariff from Mexico to the United States because what happens then if there’s a 20-percent tax placed on U.S. goods going to Mexico or anywhere else? That’s a dangerous trade war to start.”

The White House on Thursday said President Trump is considering the tax in an effort to build a southern border wall, though the administration is weighing a host of options. Trump has discussed the idea with congressional Republicans.

But there is likely to be bipartisan opposition. Just the mere mention of a tax caused an uproar on both sides of the aisle in Congress. The Trump administration quickly walked back some of its support for the plan, saying the tax is only one idea being considered in an effort to pay for the multi-billion dollar border wall.

The administration’s already-fractured relationship with Mexico was intensified this week after Mexican President Enrique Peña Nieto canceled a planned meeting with Trump. The cancellation came after Trump signed an executive order Wednesday jumpstarting the process to build the border wall, making good on a campaign promise.

Trump maintains that Mexico will pay for it, something that Mexico vehemently rejects. Trump and Peña Nieto spoke by phone on Friday, despite one of the most tense moments in U.S.-Mexico relations in years.

Beyond the export issue, lawmakers on both sides of the aisle will likely have trouble telling constituents that they supported a plan that could result in rising costs for American consumers. Business groups also are likely to oppose the effort.

“If we turn our backs on trade, it’s going to hurt our economy, and it’s going to particularly hurt Colorado,” Gardner said.

The senator did, however, appear supportive of revisiting the North American Free Trade Agreement, or NAFTA. The agreement, which has been in place for more than two decades, could face renegotiations.

Trump has signaled that he wants to dismantle the agreement, which has allowed trade to blossom, though there are concerns about accountability.

“Should we make sure that NAFTA and other agreements are being enforced the way that everybody agreed to? Yes, we should,” Gardner said. “Should we make sure that we’re entering into a bargain and that everybody is playing by the same rules we’re holding ourselves to? Yes, we should. Absolutely we should.”