President Donald Trump is talking about taking over Greenland by any means necessary. Republicans in Congress are trying to scare him back to reality.

As Trump continually threatens to bring the Danish territory into the U.S. over the objections of key global allies and the island’s elected representatives, some GOP lawmakers are stepping up their warnings and engaging in diplomacy as Democrats prepare to put the other party on record opposing a military invasion.

Sen. Thom Tillis (R-N.C.) predicted members on both sides of the aisle would lock arms and require congressional signoff if it became clear Trump was preparing imminent military action.

“If there was any sort of action that looked like the goal was actually landing in Greenland and doing an illegal taking … there’d be sufficient numbers here to pass a war powers resolution and withstand a veto,” Tillis said.

Rep. Don Bacon (R-Neb.) went further, predicting that it would lead to impeachment and calling Trump’s Greenland obsession “the dumbest thing I’ve ever heard.”

The blunt public messaging comes as lawmakers try to reassure U.S. allies, including Denmark, in private. A bipartisan group of House and Senate lawmakers will be in Copenhagen Friday to try to drive home in person the message that military action does not have support on Capitol Hill.

Senate Majority Leader John Thune is not joining the delegation but he largely endorsed the message the contingent plans to send in comments to reporters Thursday, saying “there’s certainly not an appetite here for some of the options that have been talked about or considered” — an apparent reference to military action.

The pushback amounts to one of the most profound breaches yet seen between GOP lawmakers and the president in Trump’s second term. So far the Republican uneasiness over Trump’s brash foreign policy moves have not resulted in any successful steps to restrain him.

Following the operation to remove Venezuelan President Nicolás Maduro earlier this month, five Republicans joined Democrats to advance a measure restraining Trump from future military incursions in the South American country. But on Wednesday, two of them reversed course and ended the threat after the administration made some commitments regarding future action.

Democrats believe Greenland — sovereign territory belonging to a NATO ally — could be different. Sen. Tim Kaine of Virginia, who co-authored the Venezuela measure and signaled a raft of new war-powers legislation, acknowledged to reporters Wednesday that prospects were dim that a veto-proof number of GOP senators would join Democrats’ efforts.

But “we might on Greenland,” Kaine added.

Thune’s predecessor as Republican leader, Sen. Mitch McConnell of Kentucky, spoke out in a floor speech where he said military action against Greenland would be “an unprecedented act of strategic self-harm” that risks “incinerating” NATO alliances.

Rep. Jeff Hurd (R-Colo.), meanwhile, said he was “deeply concerned” about the administration’s Greenland message.

“I don’t think it is productive, and I don’t think this is the way to treat an ally,” he said, adding that he “would be opposed to military action in Greenland.”

But even as more Republicans speak out about Trump’s Greenland ambitions, it’s not clear they could put preemptive guardrails on his actions in this Congress even if they wanted to. Instead, they appear to be hoping that Trump will read the writing on the wall and realize he doesn’t have support on the other end of Pennsylvania Avenue.

Democrats are vowing to introduce a spate of war powers resolutions, including on Greenland, in the coming weeks and months. Yet even Tillis, who predicted overwhelming support for such a resolution in the case of “imminent” military action, said he would not currently support a measure to stop Trump from using force in the region because it would “legitimize” a threat he doesn’t think is now real.

Instead, Tillis is using his megaphone as a retiring senator to launch broadsides against Trump’s top aides, whom he blames for some excesses of the administration. While a Greenland takeover might be supported by hard-line deputy chief of staff Stephen Miller, Tillis said, “it’s not the position of the U.S. government.”That, he said, is “another reason I’m going to Copenhagen.”

Sen. Josh Hawley (R-Mo.), who initially supported the Venezuela war powers resolution before backtracking, also said in an interview that he was not on board with a similar effort for Greenland.

“Not prospectively,” Hawley said, adding that any such measure “needs to respond to really particular facts.”

Any formal GOP pushback is likely to include Sen. Lisa Murkowski (R-Alaska) — co-founder of the Senate Arctic Caucus — who introduced a nonbinding resolution Thursday with Sen. Ruben Gallego (D-Ariz.), Rep. Ro Khanna (D-Calif.) and Bacon that would affirm the U.S. partnership with Greenland and Denmark. The resolution stresses the “mutual respect for sovereignty and territorial integrity” and that any military action would need congressional authorization.

Murkowski, who met with Danish diplomats this week and is also traveling to Copenhagen, said she would support a Greenland war powers resolution if it came to that. She also introduced a bill with Sen. Jeanne Shaheen (D-N.H.) that would prohibit the administration from using funding to unilaterally blockade, occupy, annex or assert control over Greenland or any other territory belonging to a NATO country.

“We are operating in times where we’re having conversations about things that we never thought even possible,” Murkowski said. “To use the name Greenland in the context of a war powers resolution is absolutely stunning.”

While a war powers resolution can be fast-tracked to the floor, Greenland’s allies in the Senate can’t easily force a vote on the NATO measure or even the nonbinding resolution. And some Senate Republicans expressed skepticism that party leaders would let those latter measures go anywhere.

“I’m sure Thune will jump on it like a bad rash,” Sen. John Kennedy (R-La.) said.

Meredith Lee Hill and Joe Gould contributed to this report.