NEW YORK (Reuters) - Major U.S. stock indexes fell on Thursday, with the S&P 500 down the most in a month, as investor worries over potential roadblocks to the Republicans’ tax overhaul more than offset optimism over strong retail sales data.

Though congressional Republicans had reached a deal on final tax legislation on Wednesday, Republican Senators Marco Rubio and Mike Lee said on Thursday they would not get behind the bill without changes to child tax credits.

As the fast-moving Republican tax revamp has evolved, it has tilted increasingly toward benefiting businesses and wealthy taxpayers, a trend that aides were saying privately is a growing concern for some lawmakers. Equity investors worry that stocks could tumble if the bill fails.

“The last catalyst left for this year is tax reform. Any market move is based on the shifting probabilities of its passage,” said Michael Antonelli, managing director, institutional sales trading at Robert W. Baird in Milwaukee, citing uncertainty over Rubio’s demands.

The Dow Jones Industrial Average .DJI fell 76.77 points, or 0.31 percent, to close at 24,508.66, the S&P 500 .SPX lost 10.84 points, or 0.41 percent, to 2,652.01 and the Nasdaq Composite .IXIC dropped 19.27 points, or 0.28 percent, to 6,856.53.

The S&P, down less than a half a percent from its record closing high hit Tuesday, posted its largest daily percentage drop since Nov. 15.

Stocks fell even as U.S. retail sales increased more than expected in November as the holiday shopping season got off to a brisk start, pointing to sustained strength in the economy.

“The fear they can’t get corporate tax cuts across the finish line might be causing the market to turn down, despite the strong retail sales and other good economic data,” said Kim Forrest, senior equity research analyst at Fort Pitt Capital Group in Pittsburgh.

Healthcare stocks .SPXHC were the biggest weight on the S&P 500, falling 1.1 percent. Some portfolio managers said investors were cashing in on recent gains in the sector.

Walt Disney DIS.N struck a deal to buy Twenty-First Century Fox's FOXA.O assets for $52.4 billion in stock.

Fox shares rose 6.5 percent to $34.88 and Disney rose 2.8 percent to $110.57, also boosting the consumer discretionary sector .SPLRCD, the only major S&P sector in the black on Thursday.

Investors were also digesting the U.S. Federal Communications Commission’s vote on Thursday to repeal landmark 2015 rules aimed at ensuring a free and open internet, setting up a court fight over a move that could recast the digital landscape.

Declining issues outnumbered advancing ones on the NYSE by a 1.89-to-1 ratio; on Nasdaq, a 2.25-to-1 ratio favored decliners.

The S&P 500 posted 21 new 52-week highs and three new lows; the Nasdaq Composite recorded 58 new highs and 63 new lows.

About 6.67 billion shares changed hands in U.S. exchanges, compared with the 6.53 billion daily average over the last 20 sessions.