* Congress to vote on tax bill afternoon onwards
* Apple slips after Instinet downgrade to neutral
* Wal-Mart, GM gain after positive broker comments
* Indexes down: S&P 0.11 pct, Nasdaq 0.29 pct, Dow 0.2 pct (Updates to open)
By Sruthi Shankar
Dec 19 (Reuters) - Wall Street's main indexes were slightly lower on Tuesday, weighed down by an Apple-led pullback in technology stocks and after two strong session of gains on rising hopes that the U.S. Congress will vote in favor of a proposed tax overhaul.
The House of Representatives will vote on the bill on Tuesday afternoon, followed by the Senate later in the night or on Wednesday morning, and will likely be signed into law by the end of the week.
The bill would cut corporate tax rates to 21 percent from 35 percent, which investors are betting will boost profits as well as trigger share buybacks and higher dividend payouts.
"There's still strong belief that it's going to happen. But the market is not going to completely relax until they get done," said Scott Brown, chief economist at Raymond James in St. Petersburg, Florida.
Apple fell 1.23 percent after broker Instinet downgraded the stock to "neutral," saying the supply/demand balance for the iPhone X suggested little space for raise estimates for the next quarter.
Apple's drop weighed the most on the S&P 500 technology sector, which dropped 0.31 percent. A half-a-percent drop in Facebook and Microsoft also dragged down the index and weighed on the market.
At 9:50 a.m. ET (1450 GMT), the Dow Jones Industrial Average was down 48.75 points, or 0.2 percent, at 24,743.45 and the S&P 500 was down 2.9 points, or 0.11 percent, at 2,687.26.
The Nasdaq Composite was down 19.95 points, or 0.29 percent, at 6,974.81. The index briefly topped 7,000-point mark on Monday for the first time, after rallying along with the broader market on rising hopes of the tax bill being passed.
Another expected outcome of lower taxes is cash repatriation. That, along with a modest rise in interest rates, will leave a lot of cash on corporate balance sheets, which makes a "very strong market for M&A," said Brown.
After a spate of dealmaking on Monday, Tuesday kicked off with health insurer Humana's $4.1 billion, or $9 per share, offer for Kindred Healthcare.
Kindred's shares fell 5.5 percent to $8.97, from its close of $9.50 on Monday after a run-up on media reports of a deal. Humana shares were down about 1 percent.
Five of the 11 major S&P sectors were higher, led by a 0.4 percent increase in the consumer staples index.
The biggest boost came from Wal-Mart, which jumped 1.51 percent after Citi upgraded the stock to "buy", on expectations of the stock will rise further in 2018.
General Motors rose 1 percent after RBC upgraded the automaker's stock to "outperform" on expectations of better profitability in 2018.
Navistar International gained more than 10 percent as higher demand for its buses and trucks drove its quarterly results.
Declining issues outnumbered advancers on the NYSE by 1,301 to 1,250. On the Nasdaq, 1,282 issues rose and 1,206 fell. (Reporting by Sruthi Shankar in Bengaluru; Editing by Savio D'Souza)