* Congress to vote on tax bill afternoon onwards
* Apple slips after Instinet downgrade to neutral
* Wal-Mart, GM gain after positive broker comments
* Futures: S&P up 3 pts, Nasdaq down 1 pt, Dow up 45 pts (Adds comment, details; updates prices)
By Sruthi Shankar
Dec 19 (Reuters) - Wall Street's major indexes were poised to inch higher at the open on Tuesday, building on strong gains in the past two sessions on rising hopes that the U.S. Congress will vote in favor of the 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. The bill will likely be signed into law by the end of the week.
"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.
The overhaul 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.
Those hopes have spurred a rally in the major Wall Street indexes this year. The Dow Jones Industrial Average, S&P 500, Nasdaq and the small-cap Russell 2000 ended at record highs on Monday.
The Nasdaq briefly topped 7,000 points for the first time and the Dow closed 208 points shy of the 25,000-point mark.
At 8:26 a.m. ET (1326 GMT), S&P 500 e-minis were up 3 points, or 0.11 percent, with 87 contracts changing hands and Dow e-minis were up 45 points, or 0.18 percent, with 30 contracts changing hands.
Nasdaq 100 e-minis were down 1 point, or 0.02 percent, in volume of 26 contracts.
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 5.8 percent to $8.95, from its close of $9.50 after a run-up on media reports of a deal. Humana shares were flat.
Apple dipped 0.8 percent after brokerage Instinet downgraded the stock to "neutral" on expectations of a limited upside to iPhone X sales next quarter.
Wal-Mart jumped 1.28 percent after Citi upgraded the stock to "buy", on expectations of the stock will rise further in 2018. The stock has gained about 42 percent so far this year, which is its best since 1999.
GM rose about 1.3 percent after RBC upgraded the automaker's stock to "outperform" on expectations of better profitability in 2018.
Darden Restaurants was up 2.3 percent, the top gainer among S&P 500 stocks trading premarket, after reporting upbeat quarterly results.
Navistar International gained more than 9 percent as higher demand for its buses and trucks drove its quarterly results. (Reporting by Sruthi Shankar in Bengaluru; Editing by Savio D'Souza)