President Trump has floated a plan to cut the State Department’s budget by an enormous 37 percent, essentially slashing diplomacy and foreign aid to make room for an $18 billion increase to defense-related spending. But members of Congress, on both sides of the aisle, are quickly shooting down the idea.

Roughly 100 congressional Democrats have signed a letter, to be delivered to Secretary of State Rex Tillerson later on Wednesday, urging him to use his clout in the administration to oppose the cuts from the inside. Key Republicans loudly oppose the cuts, with Majority Leader Mitch McConnell saying Tuesday that he was “not in favor” of cuts as deep as the ones Trump proposes.

“I am very concerned by reports of deep cuts that could damage efforts to combat terrorism, save lives, and create opportunities for American workers,” Ed Royce, the chair of the House Foreign Relations Committee, said in a statement provided to Vox. “The committee will thoroughly review the administration’s foreign affairs budget request when it’s made available to Congress.”

Trump’s huge State Department cuts seem virtually certain not to happen in their current form. And that tells us a lot about Trump’s ability to remake American foreign policy as dramatically as at least some of his advisers want to.

The Senate math is already looking grim

Trump and some of his key advisers, such as senior strategist Steve Bannon, have expressed deep skepticism about the broad arc of American foreign policy. From that point of view, it makes sense to slash State’s budget — if you think the State Department is dedicated to doing stuff America shouldn’t be doing at all, then it doesn’t really make sense to fund it at the level you currently do.

But Trump and Bannon need congressional support to radically alter State’s size and role. And very few people in Congress, on either side of the aisle, share his and Bannon’s overall view of foreign policy. For the most part, members of Congress on both sides of the aisle still buy into the traditional bipartisan consensus, which means they can use the power of the purse to check the president’s revisionist aims.

To see why Trump’s proposed cuts are in trouble, let’s take a closer look at the Senate. (House math is harder to calculate, because it’s a much larger body.) Republicans control the chamber by a thin 52-48 margin, which means that if three or more Republicans oppose a Trump initiative, its chance of passage drops dramatically.

At least four Republicans have publicly announced opposition to the level of State Department cuts the White House is talking about, which would reduce its annual budget of $50.1 billion to $36.1 billion.

Sen. Lindsey Graham had the strongest statement: “It’s dead on arrival, it’s not going to happen, it would be a disaster,” he told the Washington Post when asked about Trump’s proposal. Sen. John McCain, a close ally of Graham’s, told reporters that he’s "very much opposed” to the State Departments cuts.

Sen. Marco Rubio gave a speech on the Senate floor on Tuesday calling for maintaining the foreign affairs budget. “This idea that somehow we can just retreat from our engagement in the world is bad for national security, it's bad for our economy,” he said. “And by the way, [it] doesn't live up to the standards of who we are as a people.”

Then there’s McConnell, the most important of these three by far. If McConnell were insisting on party discipline here, Republicans like Rubio and Graham might be forced to get over their personal opposition and back the president. But by expressing his own opposition, and telling reporters the 37 percent cut “probably” won’t be able to pass the Senate, McConnell is providing cover for other Republicans to break with the president.

Now, not every important Republican has come out swinging against Trump’s proposal. Sen. Bob Corker, the chair of the Senate Foreign Relations Committee, is taking a more measured tone.

“I had a great meeting yesterday with Secretary Tillerson and will be discussing this issue with him in greater detail once the department has the opportunity to more fully review the proposed budget numbers,” Corker said in an emailed statement.

But the fact that four Senate Republicans, including the majority leader, are already so critical is about as clear a signal as possible that Trump’s proposal for slashing diplomacy and foreign aid is dead.

Institutional opposition to State Department cuts runs very deep

The State Department budget is a very easy rhetorical punching bag for conservatives, as most people don’t fully understand what it funds or the size of its budget. Some of the parts people do understand, such as foreign aid, are fairly unpopular. The average American thinks about 26 percent of the federal budget goes to aid programs, according to a 2015 poll by the Kaiser Family Foundation. The actual figure is less than 1 percent.

The problem is that experts on US foreign policy, across the political spectrum, see State’s work as absolutely vital. The House Democratic letter to Tillerson is particularly clear on why:

With instability and turmoil on the rise around the world, our international affairs efforts address the root causes of conflict and crisis. Our diplomats settle disputes over conference tables and in quiet conversation so that they don’t need to be settled with bombs and bullets. Our development initiatives help countries lift themselves up, because areas mired in poverty often become hotbeds for instability and violence. This work also shows the world that the United States believes in opportunity, equality, and the rule of law for all people. As Ronald Reagan said, “America is a shining city upon a hill whose beacon light guides freedom-loving people everywhere.” Those are the values we must continue to project across the world.

This is why, as the letter notes, 120 generals and flag officers signed a letter in February opposing cuts to the State Department’s budget. High-ranking military officials do not believe that force alone can solve US foreign policy problems. In fact, several recent secretaries of defense have explicitly called for the department to receive more money, not less, in part to reduce the demands on an overstretched military. When current Secretary of Defense Jim Mattis was a general, back in 2013, he struck a similar tune.

“If you don’t fund the State Department fully, then I need to buy more ammunition ultimately. So I think it’s a cost benefit ratio,” Mattis told members of Congress. “The more that we put into the State Department’s diplomacy, hopefully the less we have to put into a military budget as we deal with the outcome of an apparent American withdrawal from the international scene.”

The baseline idea here is that America’s existing diplomatic engagement with the world is, broadly speaking, working — that participation in international institutions like the United Nations, alliances like NATO, and development initiatives in places like sub-Saharan Africa serve America’s national interests. A lot of what the State Department does is maintain and strengthen those institutions and initiatives — the very ones aides like Bannon want to discard.

But as much power as the president may have in foreign affairs — and it’s a really a lot — there’s still a limit to how much he can change longstanding commitments in US foreign policy. The backlash to his State Department cuts is exposing those constraints very clearly.