They gave floor speeches on the importance of NATO last week. They introduced bills meant to prevent the U.S. from attacking Denmark. And several traveled to Copenhagen to meet with Danish counterparts.

But it’s not clear that will be enough, as the president continues to insist that he will take control of the Arctic island. It’s raised fears of an end to NATO — a decades-old alliance that has been a pillar of American strength in Europe and around the globe — and raised questions on Capitol Hill and around the world about what Trump’s aggressive, go-it-alone foreign policy will mean for world order.

“When the most powerful military nation on earth threatens your territory through its president over and over and over again, you start to take it seriously,” Sen. Chris Coons told The Associated Press.

The Delaware Democrat organized the bipartisan trip to Denmark to “bring the temperature down a bit,” he said, as well as further talks about mutual military agreements in the Arctic. Republican Sens. Thom Tillis of North Carolina and Lisa Murkowski of Alaska accompanied a handful of Democrats on the trip. Also, Republican lawmakers joined in meetings in Washington last week with the Danish foreign minister and his Greenlandic counterpart where they discussed security agreements.

Yet it’s clear Trump has other ideas. He said Saturday he will charge a 10% import tax starting in February on goods from eight European nations because of their opposition to his Greenland plans.

Trump said on social media that because of modern weapons systems “the need to ACQUIRE is especially important.”

The pushback to Trump’s Greenland plans

Key Republicans have made clear they think that forcefully taking Greenland is out of the question. But so far, they’ve avoided directly rebuking Trump for his talk of possessing the island.

Tillis on social media called Trumps tariff plans “bad for America, bad for American businesses, and bad for America’s allies.”

Senate Majority Leader John Thune, R-S.D., told reporters Thursday that “there’s certainly not an appetite here for some of the options that have been talked about or considered.”

In a floor speech, Thune’s predecessor as Republican leader, Sen. Mitch McConnell, warned that an attempt to seize Greenland would “shatter the trust of allies” and tarnish Trump’s legacy with a disastrous foreign policy decision.

Republican and Democratic lawmakers alike see an obvious path to bolstering American interests in Greenland while keeping the relationship with NATO-ally Denmark intact.

In a meeting with lawmakers Thursday, Danish Foreign Minister Lars Løkke Rasmussen and his Greenlandic counterpart Vivian Motzfeldt discussed how the countries could work together to develop critical mineral industries and military cooperation, Coons said. The diplomats also told the senators there is no evidence of Chinese or Russian activity in Greenland.

Trump has made the argument that the U.S. should take Greenland before China or Russia do, prompting worry across Europe. Troops from several nations have been sent to Greenland in support of Denmark.

Murkowski said on social media that “our NATO allies are being forced to divert attention and resources to Greenland, a dynamic that plays directly into Putin’s hands by threatening the stability of the strongest coalition of democracies the world has ever seen.”

What can Congress do?

Lawmakers are looking at a few options for taking a military attack on Greenland off the table. Still, the Trump administration has shown little if any willingness to get congressional approval before taking military action.

Lawmakers, including Republicans like Murkowski, are pushing legislation that would prohibit Department of Defense funds from being used to attack or occupy territory that belongs to other NATO members without their consent.

The Alaska senator also suggested Congress could act to nullify Trump’s tariffs. Murkowski and several other Republicans have already helped pass resolutions last year meant to undo tariffs around the globe, but those pieces of legislation did not gain traction in the House. They would have also required Trump’s signature or support from two-thirds of both chambers to override his veto.

Democrats have also found some traction with war powers resolutions meant to force the president to get congressional approval before engaging in hostilities. Republicans last week narrowly defeated one such resolution that would prohibit Trump from attacking Venezuela again, and Democrats think there could potentially be more Republicans who would support one applying to Greenland.

“What I’ve noticed is these war powers resolutions, they do put some pressure on Republicans,” said Sen. Tim Kaine, a Virginia Democrat who has forced votes on several similar resolutions. He said the tactic has also compelled the Trump administration to provide lawmakers with briefings and commitments to get congressional approval before deploying troops.

Still, while dismissing the Venezuela war powers resolution on Wednesday, Republican leaders made the argument that the legislation should be ruled out of order because the Trump administration has said there are currently no U.S. troops on the ground in Venezuela.

That argument may set a precedent for future war powers resolutions, giving Republicans a way to avoid voting against Trump’s wishes.

“If you don’t have boots on the ground, it’s a moot point,” said Sen. Mike Rounds, a South Dakota Republican, about war powers resolutions in general. He also argued that the prospect of taking Greenland over the objections of Denmark is nothing “more than a hypothetical.”

Other Republicans have expressed support for Trump’s insistence that the U.S. possess Greenland, though they have downplayed the idea that the U.S. would take it by force.

That’s left the strongest objections on the Republican side of the aisle coming from a handful of lawmakers who are leaving Congress next year.

Rep. Don Bacon, a Nebraska Republican, told The Omaha World Herald that an invasion of Greenland would lead to Trump’s impeachment — something he would “lean” towards supporting.

Tillis, another retiring Republican, has directed his criticism at Trump advisors like White House Deputy Chief of Staff Stephen Miller.

“The fact that a small handful of ‘advisors’ are actively pushing for coercive action to seize territory of an ally is beyond stupid,” he said.