The S&P 500 has surged about 21 percent since the election of President Donald Trump a year ago, fueled by his promises to cut corporate taxes and promises of other business-friendly measures.

Senate Republicans are unveiling a tax proposal that differs markedly on corporate, business and individual tax cuts from legislation detailed by their counterparts in the House of Representatives, Republican aides said on Thursday.

The Senate proposal delays a corporate tax rate cut to 20 percent by a year and provides small-business owners with a deduction rather than a special business rate, said the aides.

Earlier, uncertainty about the future of corporate tax rates pushed the S&P 500 down as much 1 percent, underscoring how much Wall Street is banking on a tax reduction.

The S&P 500 is trading at 18 times expected earnings, expensive compared to its 10-year average of 14.3, according to Thomson Reuters Datastream. Cutting corporate taxes would boost earnings and make stocks relatively less expensive.

"It's been a year since the election. We've gone up 22 percent on hopes of what the Trump agenda would bring, and while they're trying to work toward this thing, they haven't really accomplished much yet," said Michael O'Rourke, chief market strategist at JonesTrading in Greenwich, Connecticut.

"If progress is not made, the equity market should either pause or correct until meaningful progress is made."

Doubts about how the Senate plan might affect mortgage interest deductions hit homebuilders: Toll Brothers fell 0.86 percent and the PHLX housing index fell as much as 0.85 percent before recovering and trading down 0.13 percent for the session.

At 3:25 pm ET (2025 GMT), the Dow Jones Industrial Average was down 0.48 percent at 23,449.93, while the S&P 500 had lost 0.45 percent to 2,582.58.

The Nasdaq Composite dropped 0.73 percent to 6,739.88.

The Philadelphia Semiconductor index, a top performer in 2017, slumped 2.3 percent ahead of a quarterly report by Nvidia, which fell 2 percent.

Six of the 11 major S&P 500 sectors fell, with the technology index dropping 1 percent.

Apple, Microsoft, Alphabet, Oracle and Facebook were among the stocks weighing most on the S&P 500.

Technology has been the best performing S&P 500 sector so far this year with a 37 percent rise. The sector's stretched valuations make it vulnerable to selling as investors worry that promised tax reductions might not emerge.

Roku soared 53.5 percent after the video streaming device maker's quarterly results and guidance beat expectations.

Macy's jumped 10.87 percent after the department store operator's profit came in above expectations.

Dish Network rose 3.6 percent after the satellite and internet TV provider added subscribers in the United States in the third quarter and reduced the rate at which it lost existing customers.

Declining issues outnumbered advancing ones on the NYSE by a 2.02-to-1 ratio; on Nasdaq, a 1.82-to-1 ratio favored decliners. (Additional reporting by Tanya Agrawal and Caroline Valetkevitch; Editing by James Dalgleish)