Washington's budget fight jolted investors on Friday, reminding them that the next few weeks could bring a lot of uncertainty. Wall Street hates uncertainty.
Stocks fell in an afternoon sell-off that wiped out most of the gains from a rally earlier this week, when the Federal Reserve decided to keep its huge economic stimulus program intact.
Major indexes were mixed in morning trading, but turned lower around midday after the U.S. House of Representatives voted to defund President Barack Obama's health care law.
The vote itself wasn't a surprise, but it reminded investors that the Republican-led House and the Democratic-controlled Senate are poised for a showdown over federal spending.
The debt ceiling must be raised by Oct. 1 to avoid a government shutdown, and a potential default on payments, including debt, later in the month.
The threat of a default in August 2011 helped send global stock markets into a tailspin.
"What we've done is basically committed ourselves to two weeks of worry," said Sam Stovall, chief equity strategist at S&P Capital IQ.
Until now, September defied the worriers. The stock market has bounced backed from an August swoon, despite a calendar loaded with potential rally killers.
Fears of a conflict with Syria have faded, and Wall Street cheered when Larry Summers withdrew his name as a candidate to replace Federal Reserve chairman Ben Bernanke.
Summers, a former Treasury secretary, was viewed as more likely to rein in the Fed's stimulus program, which has kept interest rates low and boosted corporate profits.
As Middle East strife recedes from investors' minds, though, fears of budget gridlock grow.