Obviously, this could be very, very bad. That's why each side thinks the other will blink on the ACA to avoid such a calamity.
So what does the health care law have to do with any of this?
In short: a lot.
While the health care law is not directly tied to funding the government -- the CR -- or paying bills already incurred -- the debt ceiling -- it is being used as a powerful bargaining chip.
A group of Republicans, led by freshman Sen. Ted Cruz of Texas, despises the health care law so much that it is willing to risk government shutdown or default.
While 41 previous attempts to repeal, defund or stop the law haven't worked, the group hopes efforts to link it to these two pieces of must-pass legislation will. That's why there's talk of a government shutdown.
The House, which is controlled by Republicans, voted on a measure that would fund the government until Dec. 15 -- but in exchange for keeping the government open, the health care law would be defunded.
But the Democrat-controlled Senate vows that ACA defunding will have no part in efforts to keep the government running and is expected to strip that provision out of its version of the CR sometime this week.
Cruz and his supporters aren't backing down. And neither is the president.
"I believe we should stand our ground, and I don't believe (Senate Majority Leader) Harry Reid or Barack Obama should shut down the federal government," Cruz said on Fox News Sunday. "The House voted last week to fund the federal government. If Harry Reid kills that, Harry Reid is responsible for shutting down the government."
But many within the Republican Party think Cruz's idea is a terrible one. While Republicans have successfully extracted budget cuts from recent battles over government funding and the debt ceiling, most understand that a political poison pill like this is unlikely to succeed. Democrats control the Senate with their 54-seat majority.
"We are not about to shut the government down over the fact that we cannot -- only controlling one house of Congress -- tell the president that we are not going to fund any portion of this, because we can't do that," Sen. Tom Coburn, Republican of Oklahoma, said on CBS's "Face the Nation."
If Democrats win the battle over government funding and the ACA stays intact, Republicans indicated they will make similar demands when the president asks to raise the debt limit.
But Obama has repeated numerous times that he will not negotiate on the debt limit either.
"We will not negotiate whether or not America should keep its word and meet its obligations. We're not going to allow anyone to inflict economic pain on millions of our own people just to make an ideological point," Obama said at the Congressional Black Caucus awards dinner this past weekend.
And even if Cruz and his supporters somehow overcame the Democrat-controlled Senate and successfully sent a bill to the president's desk that defunds the ACA, would Obama really sign a bill that guts his signature legislative achievement?
And so we have the showdown.