The Republican-led US House of Representatives was expected to give final approval to the legislation later on Wednesday, which would enact the nation's biggest tax code overhaul in more than three decades. The House initially passed the plan but was forced to re-vote due to procedural problems.
The tax bill will slash the corporate income tax rate to 21 percent from 35 percent. Analysts say the cuts will boost business earnings and lead to higher dividends and stock buybacks for investors.
US stocks rallied ahead of the tax legislation, introduced six weeks ago, but now some investors have begun to sell.
"For the most part, the market has priced in tax cuts, there is even a risk that we might see a small pullback once the bill is signed into law," said Randy Frederick, vice president of trading and derivatives for Charles Schwab in Austin, Texas.
The Dow Jones Industrial Average rose 18.44 points, or 0.07 percent, to 24,773.19, the S&P 500 gained 0.68 points, or 0.03 percent, to 2,682.15 and the Nasdaq Composite dropped 9.44 points, or 0.14 percent, to 6,954.41.
MSCI's gauge of stocks across the globe shed just 0.01 percent.
European stock markets fell, with blue-chip indexes in Berlin, Paris and London 0.2 to 0.4 percent lower on the day. The pan-European FTSEurofirst 300 index lost 0.73 percent.
The tax plan has boosted US Treasury yields, which were at nine-month highs on optimism the plan will boost growth.
"US yields have jumped sharply higher in the last two days as the prospect of higher inflation and growth prompted some positioning adjustments in anticipation that the measures, if passed, could prompt conditions that might see rates have to rise faster than expected next year," said Michael Hewson, chief market analyst at CMC Markets in London.
Benchmark 10-year notes last fell 4/32 in price to yield 2.4771 percent, from 2.463 percent.
The 30-year bond last fell 19/32 in price to yield 2.8529 percent, from 2.823 percent.
The US dollar slid against most currencies except the Japanese yen, on expectations the tax bill's upside impact on the greenback has been factored into the market.
The dollar index fell 0.2 percent That led the euro to rise up 0.39 percent to $1.1884. The dollar also fell against sterling, the Canadian and New Zealand dollars.
The weaker dollar pushed up gold prices for a fourth day. Spot gold added 0.3 percent to $1,264.86 an ounce. US gold futures gained 0.32 percent to $1,268.30 an ounce.
Crude prices firmed modestly, supported by a larger-than-expected drop in US inventories and a continued outage in the North Sea Forties pipeline system.
West Texas Intermediate crude futures gained 20 cents to $57.79 a barrel, while Brent crude was up 39 cents to $64.20 a barrel.