When it comes to political brinkmanship, the market's story line has an unthinkable ending.

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NEW YORK — As fights go, Wall Street views the slugfest between Democrats and Republicans over the government shutdown as the undercard event. The main bout is the coming showdown over raising the debt ceiling and making sure the U.S. has enough cash to pay its bills and avoid the unthinkable: defaulting on its debt.

"The shutdown is a sideshow," says Brian Belski, chief investment strategist at BMO Capital Markets. "It's all about the debt ceiling and potential default."

There's a big difference between the hit to confidence and the economy due to the government temporarily closing for business, and the more serious threat of putting the full faith and credit of the USA at risk.

On Thursday, which marked Day 3 of the government's partial shutdown, volatility in the stock market began to rise. The Dow Jones industrial average fell more than 180 points before finishing down 137 points and below 15,000. Fears of a drawn-out fight over the shutdown have shifted to worries that Congress won't agree to bump up the nation's borrowing limit in time to avert disaster. The U.S. Treasury said it will be virtually out of cash on Oct. 17.

Still, there's a belief on Wall Street that the consequences of the U.S. not meeting its financial obligations would be so devastating to the economy and markets that there's virtually no way Congress will allow the first-ever U.S. default.

The U.S. not making timely interest and principal payments to holders of U.S. government debt is "the single most bearish scenario," says Adam Parker, chief U.S. equity strategist at Morgan Stanley. And Congress knows that.

"There is a 0% chance that the U.S. will default," Parker says.