* Indexes at session lows
* Boeing falls after Delta cancels order
* Nvidia set for worst day in December after Citron tweet
* Indexes down: Dow 0.26 pct, S&P 0.58 pct, Nasdaq 0.62 pct (Updates to early afternoon)
By Yashaswini Swamynathan
Dec 28 (Reuters) - Wall Street slipped for the first time in three days on Wednesday due to losses across sectors, hindering the Dow's march towards 20,000, a level it has never breached.
The blue-chip index came within 19 points of the milestone shortly after the open, before slipping into negative territory as losses in Boeing and Walt Disney offset Goldman Sach's 0.96 percent gain.
Boeing fell 0.8 percent to $156.26 after Delta Air Lines said on Tuesday that it had reached an agreement with the planemaker to cancel a $4 billion order for 18 Dreamliner aircrafts.
U.S. equities have been riding a post-election rally, feeding on optimism that Donald Trump's policies would be business friendly, especially to banks and industrials. The rally has also been supported by a spate of strong economic data.
The Dow, which has benefited the most among the three major Wall Street indexes from the Trump rally, is on track to have its best yearly gains since 2013. The S&P 500 is set to register a double-digit rise this year, compared with a 0.7 percent decline in 2015.
At 12:29 p.m. ET (1729 GMT) the Dow Jones Industrial Average was down 51.22 points, or 0.26 percent, at 19,893.82.
The S&P 500 was down 13.1 points, or 0.58 percent, at 2,255.78.
The Nasdaq Composite was down 34.16 points, or 0.62 percent, at 5,453.29.
"It looks like a bit of profit taking after a big move upwards which is perfectly healthy for the markets," said Adam Sarhan, chief executive officer of 50 Park Investments in Orlando, Florida.
"The fact that the Dow has not gotten above 20,000 yet shows that some investors are locking in gains."
The dollar index rose 0.55 percent and is on track for its best day since Dec. 15, helped by strong economic data on Tuesday.
All 11 major S&P 500 sectors were lower on Wednesday, with technology and consumer staples indexes being the biggest drags.
Nvidia fell 4.6 percent to $111.90 and was set for its worst day in a month after short-seller Citron Research tweeted that the chipmaker's stock was heading to $90 in the new year. The stock was the biggest drag on the S&P and the Nasdaq.
Declining issues outnumbered advancers on the NYSE by 2,098 to 801. On the Nasdaq, 1,987 issues fell and 784 advanced.
The S&P 500 index showed 11 new 52-week highs and three new lows, while the Nasdaq recorded 87 new highs and 27 new lows. (Reporting by Yashaswini Swamynathan in Bengaluru; Editing by Anil D'Silva)