The House of Representatives, as expected on Thursday approved its version of the tax reform legislation, while a key Senate panel cleared a different version.

However, the path to a majority is considered much tougher in the upper chamber of Congress than in the House. Leading analysts vary in their assessment of the odds that Washington will ultimately succeed in passing the measure.

About 20 minutes into trading, the Dow Jones Industrial Average stood at 23,309.99, down 0.3 percent.

The broad-based S&P 500 dropped 0.1 percent to 2,582.05, while the tech-rich Nasdaq composite Index shed 0.1 percent to 6,790.04.

Twenty-First Century Fox shot up 5.1 percent on reports that Comcast and Verizon are circling possible acquisitions of the film and television assets from Rupert Murdoch's media empire. Comcast lost 1.7 percent, while Verizon gained 0.8 percent.

Tesla Motors gained 2.4 percent as it unveiled an all-electric semi truck at a California event.