On Tuesday, Defense Secretary Jim Mattis and two top Republican lawmakers will meet with President Trump. Their goal, several officials told me, is to persuade him not to cut the defense budget. They will argue that this would be dangerous for national security — as well as being bad politics.

Trump often brags that he has rebuilt the military after years of neglect by President Barack Obama. Yet his White House’s Office of Management and Budget (OMB) has required that all departments — including the Pentagon — cut 5 percent from their planned budgets for fiscal 2020. In October, Trump told reporters, “We’re going to be asking for a 5 percent cut from every secretary," and predicted the defense budget "will probably be $700 billion.”

In the Oval Office Tuesday, Mattis will joined by Senate Armed Services Committee Chairman James M. Inhofe (R-Okla.) and House Armed Services Committee Chairman Mac Thornberry (R-Tex.). They want the Pentagon to be exempted from those cuts. They are fighting an uphill battle in explaining why $700 billion for the military in an age of fiscal austerity is not enough.

The Trump administration’s own plan was to increase the Pentagon’s budget from $716 billion in 2019 to $733 billion in 2020. Under the OMB plan, the Pentagon’s total down would go down to $700 billion. Inhofe and Thornberry argued last week in the Wall Street Journal that this cut would hurt military readiness and hurt the Pentagon’s efforts to prepare for threats of the future. Mattis endorsed their op-ed in a speech Saturday at the Reagan National Defense Forum in California.

“Fiscal solvency and strategic solvency can coexist,” Mattis said, referring to the Inhofe-Thornberry op-ed. He said cutting the defense budget “would be a dangerous disservice to our troops and the American people they serve and protect. We all know that America can afford survival."

While Mattis’s comments could be seen as contradicting the White House — or at least OMB — a senior defense official assured me that’s not the case, and that Mattis was actually arguing against going back to the limits of the Budget Control Act of 2011, which imposed more drastic cuts under the previous administration. The Pentagon is currently crafting a 2020 budget that adheres to the $700 billion figure, the official said.

Needless to say, Mattis isn’t opposed to more Pentagon funding. “He wouldn’t turn it down, that’s for sure,” the official said.

The goal of the Trump meeting is to lay out for the president what a $700 billion budget would mean for military readiness and modernization. Mattis’s own National Defense Strategy is based on the assumption of a $733 billion budget in 2020. Without that money, the strategy will have to be adjusted.

A senior Republican congressional aide said, in fact, Mattis, Inhofe and Thornberry are working together to go around OMB to reverse the cuts: “The objective here is to convince the president not to cut the defense budget.”

Claude Chafin, a spokesman for Thornberry, told me: “The chairman has engaged with the White House regularly on national security matters throughout the president’s term. This is just another step in that long conversation. But the chairman also feels very strongly that these kinds of conversations are best kept private.”

It’s not just about national security; politics are also at play. The incoming House Democratic leadership is already signaling that they want to cut defense spending. Rep. Adam Smith (D-Wash.), who will soon replace Thornberry as House Armed Services Committee chairman, has said so publicly many times.

Inhofe and Thornberry are just the authorizers; ultimately it will be up to the leadership and appropriators to set the budget numbers. But they successfully pushed to get the $716 billion number for 2019 against resistance from within their party. Without control of the House next year, that case is going to be even more difficult. If they can convince Trump to support their cause, that could go a long way.

Congressional Republicans think that if the Pentagon budget is going to be cut, Democrats should be the ones seen with their hands on the knife. “The headline for us is, let us not be the ones to suggest the cut,” the GOP congressional aide said. “Give us some leverage in the discussions.”

The budget numbers will only be one point of contention between Republicans and Democrats on defense next year. Inhofe and Smith have already been trading barbs, and the two are set to clash over nuclear policy, transgender troops, China and much more.

But the budget debate is crucial because it affects almost everything else. Mattis often says that relative to gross domestic product, the defense budget is low in historical terms. That’s true, but our country also faces unprecedented fiscal and economic challenges that require setting priorities, making choices and managing risk.

$700 billion is several times greater the military budget of any other country. The Pentagon can’t even pass an audit, but there’s no doubt we lose billions each year to waste, fraud and abuse.

In the end, the United States can afford whatever it takes to keep us safe. But as Todd Harrison of the Center for Strategic and International Studies points out, “The problem for defense is not the size of the budget, it’s how the budget is spent.”