Some years back a hit song filled the summertime airwaves with its chorus of “See You In September.”

It was meant to be a lover’s promise of joyful reunion at summer’s end.

But to use those words in Washington, D.C., right now sounds more like a warning … or even a threat.

That’s because when Congress returns after Labor Day and official business resumes, the agenda will be heavy, the deadlines tight and the atmosphere fraught with potential fiscal disaster.

The end of the fiscal year comes on Sept. 30. That’s the deadline for the spending bills for the new fiscal year. And that homework didn’t get done before Congress left on its August recess.

Those spending bills really matter because without them the government starts to shut down on Oct. 1. We have seen this happen as recently as 2013, with substantial consequences and overwhelmingly negative public reaction.

Polls show the public is in no mood for a replay. Those polls held after President Trump in August said he wanted his Mexican border wall to be funded even if it triggered a shutdown crisis. Trump vowed that his signature issue, the wall, will be financed somewhere in the welter of legislation awaiting action this month and said he was willing to have a shutdown rather than see the wall abandoned. Since that threat was issued in late August, some reports have suggested he would not make good on it. But White House press secretary Sarah Huckabee Sanders insisted on Friday that Trump still meant the wall-or-shutdown ultimatum as stated.

But this is not just a crisis about future spending. It’s also about 200 years of accumulated debt to pay for past spending. The federal government has reached (and breached) the ceiling Congress most recently placed on its authority to borrow money — the number people call the “debt limit.”

The debt limit matters because in a few weeks the Department of Treasury will exhaust its delaying tactics and be unable to issue new debt. Without new debt to pay off old bonds and other obligations coming due, Treasury says it would have to default.

What would happen then? The U.S. has never defaulted, so we don’t know for certain. But the general consensus among economists and investors is that bad things would happen. Very bad things. More about this in a moment.

First, we should note that Congress also has other urgent items to attend to in September. By far the most pressing will be the emergency measures to help the people of Houston and East Texas who have been devastated by Harvey, a historic storm.

The Trump administration has asked for nearly $8 billion in emergency funding, a down payment on an aid package that many expect to reach $100 billion or more. That will be a lot for fiscal conservatives to swallow, especially if no offsetting cuts to other kinds of spending can be enacted.

The September Congress also has a deadline looming for the reauthorizing of the Federal Aviation Administration, one of those pillars of government service people take for granted most of the time.

Congress has been delaying this particular chore because of deep disagreements about the mix of private and public authority involved in American air travel. Not all of that conflict will be resolved in the coming days, but deals must be struck to keep the nation’s commercial air traffic in the air.

But back to the fiscal collision one White House official has called “brutal.” What do we really have to worry about in all this? Can’t Congress just get down to business and get this stuff done?

Everyone wants to move on to President Trump’s grander wish list, including an infrastructure bill (jobs and projects galore) and the rewriting of the tax laws (which is assumed to include some sweet tax cuts for businesses and individuals alike).

But right now those issues look like dessert. First, Congress has to clean its plate.

The first forkful should probably be a budget resolution. This is useful not only to guide the spending process, but to set up the debate on taxes. Without a budget resolution, Senate Republican leaders would not be able to rule out filibusters when they got around to considering a tax overhaul. That means they would need 60 votes instead of 50 to prevail. (And on health care recently, they could not even get 50.)

How hard is it to pass a dozen spending bills for programs people generally want to have funded? Harder than you think. Parts of Congress have been working on just this problem all year without making much headway.

The House did manage to pass four of its 12 spending bills before going on August recess. But the Senate has neither taken up these four House offerings, nor passed any of its own. Zero.

So let’s do the math. We need eight more spending measures from the House and a dozen from the Senate. Then all those bills would need to be mashed up in conference committees between the two chambers. The results would still need to pass both chambers and be signed by President Trump.

Sure, deadlines make a difference. But they don’t make disagreements and partisan blood feuds disappear. So…happen overnight… this will not.

You might take solace in the fact that Congress still has almost a month to get all this done. Except that leadership has only scheduled a dozen legislative working days in September. So, right now it seems a safe bet we will need that old reliable tool from the days of divided government. It’s known as a “continuing resolution,” and it will keep the government operating past Oct. 1. We will probably need more than one CR before it all gets done.

That would at least forestall the prospect of a shutdown over spending. But the greater threat at this point might be coming from that other problem: the debt limit and the specter of default.

Default is different from shutdown. We have survived shutdowns several times, in part by exempting such essentials as the military, emergency relief such as for Harvey, air traffic control and meat inspection.

But default is unknown territory. It’s never happened. That may make some people say: “Let’s try it.” But those might not be the people you want to trust with your life savings.

If Treasury defaults on U.S. obligations, the reliance on the dollar and on U.S. debt instruments as the ultimate safe haven for creditors and investors all over the world would be at risk. Credit markets would absorb the worst shock since the crisis of 2008 that triggered the worst recession since the 1930s.

At a minimum, the cost of borrowing more money in the future would go up. Probably by a lot. And those higher interest costs would add more to the annual deficit, and the national debt.

That is why default is usually not even discussed and why the debt limit must be raised.

Congress first imposed the debt limit 100 years ago to mollify anti-debt (and anti-war) elements among its members as the U.S. entered World War I. The idea was to tell Uncle Sam he could go a billion into debt to defeat the enemy, but no more.

That amount seems laughable now. The limit has been lifted many times since, by ever escalating amounts. The current U.S. debt stands at about $20 trillion. Even in the current low-interest rate environment, paying the interest on the debt is one of the larger items in the federal budget — after things like defense and security, Social Security, the major federal health insurance programs and safety net programs.

And of course, many Republicans in Congress have campaigned on slashing federal spending and reducing the federal debt without raising taxes. Many have said they will not vote for a debt limit increase ever. Others say they won’t support one without deep spending cuts attached.

Yet the debt limit must be raised. House Speaker Paul Ryan, R-Wis., and Senate Majority Leader Mitch McConnell, R-Ky., have said they will do so, knowing they cannot make good on their pledge without Democratic help. In the House, especially, Ryan will probably need quite a few Democratic votes to make up for the defections in GOP ranks. Will the Democrats do it? Yes, but at a price steep enough to make the Republicans blanch.

Can Ryan find the sweet spot where he cuts enough spending to get enough Republicans but not so much as to sacrifice his slice of the Democrats?

The Trump administration has called for a clean debt limit bill, meaning a substantial increase in the debt limit with no changes to spending or taxes or anything else. Some may want Trump to attach his border wall to the debt limit. But the inner circle of his financial and economic advisers — such as Wall Street alumni Steve Mnuchin and Gary Cohn — regard this as anathema.

Will they prevail? Will the votes be there? If not, as Treasury’s cash flow peters out, one solution could be to shut down parts of the government.

Some conservatives have argued that a partial shutdown would save enough money to enable the Treasury to meet obligations coming due — rather than default. But Treasury officials see it differently. They say the day is coming when default will be real.

Which is one more big reason why “see you in September” now means that a day of reckoning has arrived.