Washington (CNN) Republican lawmakers voicing their alarm at President Donald Trump's threat to impose escalating tariffs on Mexican goods find themselves once again in the uncomfortable position of having to answer for Trump's trade impulses they fundamentally disagree with.

Yet it's unclear whether a Congress that has consistently stood idly by during Trump's previous -- and ongoing -- trade wars will act to prevent any new tariffs from taking effect.

Trump said Thursday night that he would use a national emergency authority to slap tariffs of 5% on all imports from Mexico to the US , increasing to 25% in October if the Mexican government doesn't act to curb illegal border crossings.

Since Trump came into office, GOP lawmakers have privately and publicly urged the President to stand down on imposing tariffs on America's closest trading partners, but have never acted to prevent it. After Trump lifted steel and aluminum tariffs on Canada and Mexico earlier this month, members of Congress breathed a sigh of relief, hopeful he would prioritize ratification of his renegotiated North American Free Trade Agreement and avoid any new trade wars amid heightened tensions with China.

Those hopes were dashed Thursday, when Trump announced potential tariffs on Mexico, even as the Mexican legislature was preparing to soon consider the US-Mexico-Canada trade agreement.

The vast majority of Republicans fear the tangible effects of the tariffs -- harm to American businesses who depend on goods from Mexico , as well as higher prices for US consumers on a large scale. But because Trump is invoking border security, GOP lawmakers are caught between affirming his immigration stance and condemning his drastic approach to trade.

"I support nearly every one of President Trump's immigration policies, but this is not one of them," Senate Finance Committee Chairman Chuck Grassley of Iowa said in a statement Thursday.

Some Republicans on Capitol Hill are still holding out hope Trump could reverse course. If he doesn't, lawmakers may consider legislative options to block the new tariffs. Those conversations will likely take place next week, after Congress returns from a week-long recess.

But they could also do what they've done all along: cross their fingers and hope for the best.

The White House said in a statement that the tariffs would be deployed June 10 using authority under the International Emergency Economic Powers Act, a law that has never been used for tariffs, and it is uncertain Trump's broad application of the power could hold up in court.

Republican Sens. Joni Ernst, Rob Portman, John Cornyn, Martha McSally and others also pushed back on the idea, arguing it would be painful for their states and disruptive for the North American economy.

Meanwhile, South Carolina Sen. Lindsey Graham endorsed the tactic. "I support President Trump's decision to impose tariffs on Mexico until they up their game to help us with our border disaster," he tweeted

Senate Majority Leader Mitch McConnell, for his part, carefully avoided criticizing Trump and tiptoed around the tariff question.

"There is a serious humanitarian crisis at our southern border, and it is past time for my Democratic colleagues to finally get serious about meaningful action," he wrote in a statement. "As our third biggest trading partner, a healthy and vibrant economic relationship with Mexico is a vital source of our joint prosperity. Any proposal that impacts this relationship deserves serious examination and I look forward to discussing this plan in greater detail with my colleagues and the administration."

Opponents warn the move would hurt American consumers and undercut the administration's efforts to ratify the USMCA . US Trade Representative Robert Lighthizer, along with Treasury Secretary Steven Mnuchin, urged against the tariffs beforehand, but more protectionist minds -- Stephen Miller and Peter Navarro -- prevailed.

White House press secretary Sarah Sanders also rejected any notion that Republicans should feel blindsided by the White House's move.

"The President didn't blindside his own party. If Republicans weren't aware, then they haven't been paying attention," Sanders said on Friday. "Anybody in this country or frankly in the world that says that there surprised by this has been living under a rock and not paying attention. The President's been crystal clear that we have to take action, we have to step up, we have to do more, and we have to secure our borders."

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What can Congress do?

Vocal critics of Trump's use of tariffs say Congress should reclaim trade powers from the executive branch.

"The President's use of tax hikes on Americans as a tool to affect change in Mexican policy is misguided," Pennsylvania Sen. Pat Toomey said in a statement. "It is past time for Congress to step up and reassert its Constitutional responsibility on tariffs."

Toomey has introduced a bill to require congressional approval for a different authority, Section 232 of the Trade Expansion Act, which Trump has used to impose tariffs on national security grounds in the past. Grassley has been working on a compromise plan to curb the same power. Because Trump is using a separate tool to pursue the tariffs on Mexican goods, lawmakers may consider rewriting or expanding those bills in order to address the looming threat.

Lawmakers could also end the national emergency declaration underpinning the tariffs with a joint resolution of disapproval, as they attempted earlier this year when Trump used national emergency powers to seize funding for border security. That effort failed because Congress failed to override Trump's veto.

Congress could also sit back and wait for the courts to weigh in: The White House is expected to face challenges from American business groups.

The US Chamber of Commerce said Friday it was exploring legal options to stymie the threat. During a call with reporters, Neil Bradley, executive vice president and chief policy officer at the Chamber, slammed the decision and said the group was doing its due diligence.

"Imposing these tariffs will make all the things we need to do much harder while weakening our economy at the same time," he said.