Unless Congress and the Trump administration get their act together, the federal government will shut its doors this Friday. But as bad as that sounds, it will just a be a dress rehearsal for the real challenge coming this summer.

That would be raising the federal debt ceiling.

Let's take the dress rehearsal first. To authorize the government's various agencies to keep spending money, Congress has to regularly pass budgets. If it can't agree on a budget, or temporary stopgap measures, a shutdown results.

That really should be easy to avoid: Passing a budget doesn't preclude more laws to reform health care, rework the tax code, or whatever else Congress wants to do. Moreover, the alternative is politically embarrassing: "Even our most recalcitrant members understand that if you shut down the government while you're running it and you control the House and the Senate, you can't blame anybody but yourself," Rep. Tom Cole (R-Okla.), a senior member of the House Appropriations Committee, told Reuters.

Yet Trump and Congress could well blow it.

First off, passing budgets has gotten harder as the legislature has become more partisan and perpetually deadlocked. But the problems are just as bad within the GOP: Republican leadership ought to be able to use reconciliation to pass a budget through the Senate without having to negotiate with Democrats to avoid a filibuster. But Republicans in the House in particular are such a fractious mess they may not be able to hold a majority together within their own ranks.

So negotiations seem inevitable. And the Democrats are going to have demands, like maintaining federal funding for Planned Parenthood or protecting ObamaCare's subsidies.

That's tricky enough. But according to Politico, the White House is making things much worse.

Team Trump is borderline obsessed with the symbolic marker of the president's first 100 days, particularly the fact that they have almost nothing to show for it. So they might demand funding for a border wall, or more money for defense, or fiscal punishment for so-called "sanctuary cities," as conditions for passing a budget. They're also arm-twisting GOP leadership to try to pass a new version of TrumpCare, likely even more politically poisonous than the first version.

Passing a budget in four days under any circumstance is tough. Passing one through the three-way gauntlet of GOP dysfunction, Democratic conditions, and Trump's demands will be a complete nightmare. Doing all that while trying to pass TrumpCare 2.0 will be straight-up impossible.

So there's a very good chance the GOP can't pull it off and the government shuts down until they get their act together.

Now, as humiliating as another government shutdown would be, its economic consequences are limited. Huge portions of the government (like entitlement programs) operate on autopilot even during a shutdown.

But not raising the debt ceiling is another matter entirely.

The debt ceiling is a statutory limit on how much debt the Treasury Department can have on its books. As the federal government's debt load increases, the ceiling must occasionally be raised by Congress.

Technically speaking, this is a completely superfluous process. How much debt we will or won't take on is already determined by Congress through taxation and spending law. All the debt ceiling does is allow Congress to renege on its own policy decisions. But one reason the federal debt grows is because we take on new debt to keep paying interest on old debt. So if Congress ever refuses to raise the debt ceiling, what it's really doing is refusing to honor America's debt obligations.

This would be calamitous.

U.S. debt is considered the safest asset in the global financial market. Countless other assets and investments around the globe are all keyed off it. Countries stockpile U.S. treasuries in order to maintain currency reserves, because the overwhelming majority of international financial transactions are done in American dollars. All of this hinges on people's trust that the U.S. government will abide by its promises. Take that away, and you're yanking the cornerstone out of the entire global financial system. Moreover, if the debt ceiling isn't raised, all government day-to-day spending would have to immediately fall until it was even with day-to-day revenues. Sucking that much spending out of the economy at once would induce an immediate national recession.

In 2011, the government merely flirted with not raising the debt ceiling. America's borrowing costs jumped and the economy took a noticeable hit. No one has any idea what would happen if we actually took the plunge. It could be a national or even global catastrophe.

Technically, we breached the ceiling in March. But Treasury has certain "extraordinary measures" it can use to shift money around and keep paying the government's bills. That life line will last us until mid-summer.

So why is the government shutdown a dress rehearsal for the looming debt ceiling fight?

Part of it is just the increased rancor between the parties, and the general nihilism in Congress. For Republicans or anyone else who postures about the federal debt, demagoguing the debt ceiling is too tempting to resist. It's also the ultimate bargaining chip: You can demand whatever you want in exchange for a hike, because not hiking threatens Armageddon.

But the real X-factor here is Trump himself. During the previous debt ceiling fights in 2011, 2013, and 2015, we had Barack Obama in the White House. Say what you will about his politics, Obama was incontestably a cool customer, ceaselessly willing to negotiate and to remove his own ego from proceedings. Trump is the polar opposite: fickle, incoherent, and driven by a lizard-brain need for self-aggrandizement. Instead of helping Congress negotiate through the threat of a government shutdown, Trump has already made the situation far worse by inserting his own capricious demands.

What if Trump himself realizes that taking on the debt ceiling could finally get him the wins he craves: repealing ObamaCare, passing the infrastructure bill, cutting taxes for the wealthy, etc?

The debt ceiling is a nuclear bomb built into the foundation of U.S. fiscal policy. And Trump is the lunatic we've tasked with defusing it.

Buckle up everyone.