The Dow Jones industrial average closed down 89.24 points to 12,878.13. The Dow and other indexes had been moving between small gains and losses for most of the day, then turned lower after Reid's comments to reporters in the early afternoon.
"We have to get away from the happy talk and start talking about specific things," Reid said.
The Standard & Poor's 500 fell 0.52 percent to 1,398.94 and the Nasdaq composite index fell 0.30 percent to 2,967.79.
Worries about the budget talks have been hanging over the stock market for weeks. Stocks slumped immediately after the Nov. 6 election over concerns that politicians would be unable to reach a deal to trim the deficit before a Jan. 1 deadline.
If that deadline isn't met, under current law a series of sharp tax increases and spending cuts will come into effect. Economists have warned that the measures could push the economy back into a recession.
"The markets are getting whipped around, rather sharply, on headlines," said Sal Arnuk, co-founder at Themis Trading. -- AP