(Reuters) - U.S. stocks dipped on Thursday as political uncertainty in Washington kept investors cautious ahead of comments on monetary policy from central bankers gathered for their annual meeting in Jackson Hole, Wyoming.

Speeches by Federal Reserve Chair Janet Yellen and European Central Bank President Mario Draghi on Friday will be scrutinized for hints on the path of monetary policy, but neither of them is expected to give fresh guidance.

The focus on central bankers’ views will be a departure from the past two weeks, when the stock market was roiled by concerns over geopolitics, mayhem in Washington, and President Donald Trump’s controversial comments.

President Donald Trump picked a new fight with fellow Republicans whose support he needs to advance his policy agenda, saying congressional leaders could have avoided a “mess” over raising the U.S. debt ceiling if they had heeded his advice.

U.S. House of Representatives Speaker Paul Ryan later said Congress will pass legislation to raise the federal debt ceiling and lawmakers have a number of options for avoiding default.

“There are two stories that play out in Washington these days and to a degree it does impact the market - can the President have any success with the overall Washington agenda, whether it is the debt ceiling or tax reform - so far it seems like that is not going to be simple,” said Phil Blancato, CEO of Ladenberg Thalmann Asset Management in New York.

“On the other side, there really is a necessity to see some tax reform to help the greater economy.”

Recent economic data painted a mixed picture. The number of Americans filing for unemployment benefits rose less than expected last week, while home resales unexpectedly fell in July to their lowest monthly level of the year.

The Dow Jones Industrial Average .DJI fell 28.69 points, or 0.13 percent, to 21,783.4, the S&P 500 .SPX lost 5.07 points, or 0.21 percent, to 2,438.97 and the Nasdaq Composite .IXIC dropped 7.08 points, or 0.11 percent, to 6,271.33.

Oil prices slipped amid concerns over demand. U.S. Gulf Coast refineries shut operations as Hurricane Harvey was forecast to turn into a major hurricane.

Consumer staples .SPLRCS, down 1.3 percent, were the worst performing of the 11 major S&P sectors, led lower by a 9.5 drop in J.M. Smucker SJM.N after its posted disappointing results and lowered its earnings forecast.

The group also lost ground after Amazon.com Inc AMZN.O said it plans to complete its $13.7 billion acquisition of Whole Foods Market Inc WFM.O on Monday after winning antitrust approval from U.S. regulators. The announcement weighed on grocery store stocks such as Kroger Co KR.N, off 8.10 percent, and Wal-Mart Stores Inc WMT.N, down 2.03 percent.

Dollar Tree DLTR.O advanced 5.6 as one of the best performers on the S&P 500 after the retailer's profit and comparable sales beat estimates.

Signet Jewelers SIG.N surged 16.7 percent after the company issued results and said it would buy an online jeweler.

Advancing issues outnumbered declining ones on the NYSE by a 1.02-to-1 ratio; on Nasdaq, a 1.47-to-1 ratio favored advancers.

About 5.27 billion shares changed hands in U.S. exchanges, below the 6.08 billion daily average over the last 20 sessions.