* Gary Cohn resignation rumors knocked down
* Wal-Mart drops after reporting margin fall
* Barcelona incident adds to investor worries
* Indexes down: Dow 1.2 pct, S&P 1.5 pct, Nasdaq 1.9 pct (Updates to close)
Aug 17 (Reuters) - U.S stocks sold off on Thursday, with the S&P 500 recording its biggest daily percentage drop in three months as escalating worries about the Trump administration's ability to push through its economic agenda rattled investors.
The benchmark index also closed at its lowest level since July 11, with the day's move marking the first time since the Nov. 8 election of two days with more than 1 percent declines so close together. The index dropped 1.4 percent last Thursday, as concern over a possible conflict between the United States and North Korea hit the market.
The falls mark a break from a period of low volatility and subdued moves. The S&P 500 has had just four 1 percent declines this year.
Investors appeared to be losing faith in the Trump administration's ability to move forward on its agenda, some strategists said.
"Continued problems with Republican leadership, inability to get anything done, and this latest wedge being driven between the president and Congress. Not helpful," said Stephen Massocca, senior vice president at Wedbush Securities in San Francisco.
With valuation levels considered high in many stocks, investors may be more prone to sell, he said.
The market was also on edge after a van crashed into dozens of people in the center of Barcelona and Spanish media, citing police sources, said at least 13 people were killed. Police said the crash was viewed as a terror attack.
Stocks began to lose ground early in the session, following speculation about the possible departure of National Economic Council director Gary Cohn.
A White House official later said Cohn intends to remain in his position. After a short respite, the market continued to sell off and picked up the pace into the close. All 30 Dow stocks fell, along with all components of the S&P 100.
Speculation of Cohn's departure came a day after Trump disbanded two business councils, with several chief executives quitting in protest over his remarks on white nationalists.
The Dow Jones Industrial Average ended down 274.14 points, or 1.24 percent, to 21,750.73, the S&P 500 lost 38.1 points, or 1.54 percent, to 2,430.01 and the Nasdaq Composite dropped 123.20 points, or 1.94 percent, to 6,221.91.
The S&P 500 was on pace for its worst back-to-back weeks since the election.
Disappointing corporate results also weighed. Shares of Dow component Cisco Systems fell 4 percent a day after its results, while Wal-Mart was down 1.6 percent after the retailer reported a drop in margins due to continued price cuts and e-commerce investments.
Small-cap stocks also sold off. The Russell 2000 index , a widely used gauge of small caps, closed below its 200-day moving average for the first time since June 2016.
Declining issues outnumbered advancing ones on the NYSE by a 4.55-to-1 ratio; on Nasdaq, a 3.89-to-1 ratio favored decliners.
The S&P 500 posted 49 new 52-week highs and 10 new lows; the Nasdaq Composite recorded 98 new highs and 85 new lows.
It was the seventh straight day in which the NYSE and Nasdaq had more stocks making new 52-week lows than highs, the longest stretch since Trump's election.
About 6.7 billion shares changed hands on U.S. exchanges. That compares with the 6.3 billion daily average for the past 20 trading days, according to Thomson Reuters data.
(Additional reporting by Sruthi Shankar and Tanya Agrawal in Bengaluru; Editing by Nick Zieminski and Dan Grebler)