The S&P 500 and the Nasdaq were slightly higher on Monday as a raft of deals lifted sentiment, even as investors remained cautious ahead of the outcome of the U.S. election next week.
The Dow Jones Industrial average was little changed, weighed down by a 3 percent fall in Nike following a BofA Merrill Lynch downgrade.
The FBI's review of newly discovered emails related to Hillary Clinton's use of a private server added to uncertainty over U.S. presidential elections.
While Clinton had opened a recent lead over her unpredictable Republican rival Donald Trump in national polls, it had been narrowing even before the email controversy resurfaced.
An ABC News/Washington Post poll released on Sunday showed Clinton with a statistically insignificant 1-point national lead.
"The narrowing of the polls is making the market a little bit nervous because it had priced in a Clinton victory," said Thomas Wilson, senior investment manager at Brinker Capital, Berwyn, Pennsylvania.
"This has caused some uncertainty and we all know the market hates that. I expect the market to be stuck in a no-man's land until the election."
Despite the rise on Monday, the market is headed for its worst monthly decline since January.
Dow component General Electric was up 0.38 percent at $29.33 after the company said it would merge its oil and gas business with oilfield services provider Baker Hughes. Baker Hughes was down 4.7 percent at $56.35
Level 3 Communications rose 5.1 percent to $56.78 after CenturyLink said it would buy the company in a deal with an equity value of about $24 billion. CenturyLink fell 12.4 percent to $26.63
At 12:42 p.m. ET (1642 GMT) the Dow Jones industrial average was up 19.24 points, or 0.11 percent, at 18,180.43, the S&P 500 was up 5.38 points, or 0.25 percent, at 2,131.79 and the Nasdaq Composite was up 11.15 points, or 0.21 percent, at 5,201.26.
The energy index slid 0.77 percent after oil prices fell, with the Brent crude futures hitting their lowest in more than a month.
Eight of the 11 major S&P sectors were higher, with the rate-sensitive utilities index's 2.14 percent rise leading the advancers.
The market is watching the outcome of the U.S. Federal Reserve meeting, which begins on Tuesday. The Fed is unlikely to make a move this week, as it is too close to the election, with many market participants instead expecting a hike in December.
Lumber Liquidators fell as much as 15.5 percent to $15.62, its biggest percentage drop in 6 months, after the company said it could not provide a timeline for potential settlements with regulators, or an outlook for its business.
Advancing issues outnumbered decliners on the NYSE by 1,590 to 1,263. On the Nasdaq, 1,349 issues fell and 1,341 advanced.
The S&P 500 index showed seven new 52-week highs and five new lows, while the Nasdaq recorded 31 new highs and 88 new lows. (Reporting by Tanya Agrawal; Editing by Sriraj Kalluvila)