* Alphabet falls after YouTube ad row; top drag on S&P, Nasdaq
* Accenture falls after quarterly profit slips
* Indexes: Dow -0.07 pct, S&P -0.19 pct, Nasdaq -0.18 pct (Updates to late afternoon, adds details on Trump)
By Noel Randewich
March 23 (Reuters) - Wall Street fell on Thursday as investors worried about a delay in a vote on a healthcare bill that is seen as President Donald Trump's first policy test.
The U.S. House of Representatives canceled Thursday's vote on the healthcare plan, according to a senior House Republican aide.
Failure to pass the American Health Care Act would cast doubt on Trump's ability to deliver other parts of his agenda that need the cooperation of the Republican-controlled Congress, including ambitious plans to overhaul the tax code and invest in infrastructure.
"Today's action is basically speculation about whether the House is going to pass this healthcare bill," said Bruce Bittles, chief investment strategist at Robert W. Baird.
But Bittles predicted that, even if the bill fails, any selloff in stocks will be short-lived as investors who have missed out on a rally since November jump into the market.
The S&P 500 has gained 10 percent since the election, spurred mainly by Trump's campaign promises to enact legislation seen as pro-business.
The benchmark index is trading at about 18 times expected forward earnings, compared with a 10-year average of 14, according to Thomson Reuters data.
The Dow Jones Industrial Average fell 14.89 points, or 0.07 percent, to 20,646.41, the S&P 500 lost 4.21 points, or 0.18 percent, to 2,344.24 and the Nasdaq Composite dropped 9.27 points, or 0.16 percent, to 5,812.37.
Additional reporting by Tanya Agrawal in Bengaluru; Editing by Nick Zieminski