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(CNN) —  

Congressional Republicans came back to Washington Monday night trying desperately to grapple with how to handle President Donald Trump’s threat to slap unilateral tariffs on Mexico.

The White House is working behind the scenes to calm unrest among GOP senators as members consider legislative options to push back on the tariffs. But as of now, there is no clear path forward, and Republicans are uncertain what sort of legislative recourse would be legally viable.

Trump said on Tuesday that he expects the tariffs to go into effect as promised, starting next week, and warned Republicans not to intervene.

“The threat is out there but we haven’t really started yet. This will take effect next week,” Trump said at a news conference in London.

Trump said if Republicans worked to block the tariffs they would be mistaken.

“I don’t think they will do that, if they do it’s foolish,” he said.

The Republican caucus will meet Tuesday for its weekly closed-door policy lunch, which will include representatives from the White House who will defend Trump’s decision. GOP leaders will get their first full read on how widespread the unease – and the desire to do something about it – actually is.

GOP sources told CNN Monday night Republicans were considering the possibility that they may have to vote on the national emergency underlying Trump’s threatened tariffs on Mexican goods. The vote could block the tariffs and disrupt funds the President already co-opted for border security.

While some senators have discussed options, it doesn’t mean there’s a serious effort underway to bring forward such a vote. Some of the conversation actually stems from a reluctance among senators to hand Trump a public rebuke on the matter.

The reality, first reported by Politico, is that Republican lawmakers are working through legal questions raised by Trump’s threat to use the International Emergency Economic Powers Act to impose tariffs on Mexico, with some wondering if it will set up another vote on a resolution of disapproval regarding Trump’s declared national emergency at the border.

Aides say it is possible Trump could have to declare another national emergency, separate from the one he declared earlier this year to seize funds for border security, to impose tariffs under IEEPA. If so, Congress may be able to vote again on a privileged resolution of disapproval to reject the national emergency and block the tariffs.

If a disapproval vote happens, it would be because Democrats move first

03:12 - Source: CNN
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When Congress passed a disapproval resolution for Trump’s national emergency move earlier this year, it was because House Democrats forced the question, not because Senate GOP leaders had any desire to hold a vote on the subject. The same pattern would likely play out here as well.

A dozen Republican senators split with the White House to support the resolution at the time. Whether or not the added consideration of tariffs on Mexico would build more support for the resolution within the party is an open question. Aides emphasized that because a redux of the disapproval effort would not be specific to just the tariffs, any resolution may end the exact same way: with Congress initially passing it and then deferring to Trump after a veto.

Outside business groups are keenly aware of this potential strategy and can be expected to pressure lawmakers to pursue it on the Hill.

Republicans don’t want to take a messy, public vote on this

02:35 - Source: CNN
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Republicans in Congress are not itching to cast votes against Trump on this issue.

To be sure, many members are alarmed by the White House’s strategy. But they are urging Trump to change his mind and to make a deal with the Mexican delegation when they meet with administration officials to discuss the tariffs on Wednesday.

“I’m hoping that these meetings on Wednesday will kind of disarm that situation and maybe the Mexican government will figure out some things that they can do that would prevent this kind of plan from ever getting executed,” Senate GOP Whip John Thune told CNN Monday night.

“I’m not sure what form that would take,” Thune said of a response from Congress, but if the situation escalates, “you’re going to get a lot of people very interested in this.”

Iowa Republican Sen. Joni Ernst also indicated there was no effort to prevent the tariffs yet. She said it was “quite possible” that Congress could take action if Trump follows through on the threat, “but we need to sort through the legalities of that, and that’s one thing that we’re trying to figure out as well.”

Senate leaders are telling members to reach out to Trump directly to ask him to reconsider this week.

But there is also a surprising level of support – at least publicly – for the President’s tactic among GOP senators. On Monday night, several Republicans, including Sens. Richard Shelby of Alabama, Marco Rubio of Florida and Thom Tillis of North Carolina, defended the President’s approach and sympathized with his frustration regarding border security.

Others, like Kansas Republican Sen. Pat Roberts, told reporters the plan was “extremely counterproductive.”

This is all preliminary

GOP aides emphasized they have no idea what will happen because Trump hasn’t sent an executive order or a proclamation about the issue to Congress yet. The question depends on how the administration drafts the order. Before then, a lot of these conversations are hypothetical.

On Monday, even Trump’s loudest critics on trade within the party were unsure how to block the move.

“I’ve got to understand exactly what the options are, and I haven’t had a chance to really drill down on those,” Pennsylvania Republican Sen. Pat Toomey told reporters.

CNN’s Kevin Liptak contributed to this report.