It was day one of a "Brexit"-focused week and the higher chances of a vote to stay in the European Union had global markets in a celebrating mood.

The S&P 500 was up 0.6%, the Dow Jones Industrial Average rose 0.7%, and the Nasdaq climbed 0.8%.

Weekend polls showed sentiment shifting toward a "remain" vote in the upcoming referendum on Thursday, a welcome development that brought relief to many worried over an "exit" disruption. A Survation/The Mail survey showed 45% of voters wish to "remain" while 42% prefer to "exit." Markets are expected to remain choppy until the outcome of the referendum is announced.

The chances of an exit from the EU have sent global markets into a spin, triggering fears over economic and political stability in the region.

"Potential 'Brexit' brings a litany of risks along with it, including currency, trade, and economic growth," said Jason Pride, Glenmede director of investment strategy. "Headline risks associated with an exit include the possibility of U.K. bond ratings being affected and the uncertainty surrounding the British pound. Furthermore, many have noted that the U.K. would need to renegotiate its trade agreements which could have adverse effects on U.K. economic growth."

European markets surged on the increasing likelihood of a "remain" vote on Thursday. Germany's DAX jumped more than 3%, the CAC 40 in France rocketed 3.5% higher, and the FTSE 100 in London spiked 3%.

Crude oil joined in on a global rally on Monday. The commodity posted a weekly loss on Friday as oil prices pulled back from recent attempts to recapture and remain above $50 a barrel. West Texas Intermediate closed up 2.5% at $49.19 a barrel on Monday, its highest close since June 9.

"The petroleum markets have extended Friday's gains on a further downgrading of the risk that Britain will vote to leave the European Union in Thursday's referendum, with stronger equities and a weaker U.S. dollar helping to drive a likely combination of short covering and risk-on trade flow in the crude oil market," said Tim Evans, energy futures analyst at Citi.

The mega merger between Anthem (ANTM - Get Report) and Cigna (CI - Get Report)  looked less likely following reports U.S. antitrust regulators could impede their proposed $48 billion deal. Regulators are reportedly skeptical that the health insurers can provide concessions that would ensure the industry remains competitive, The Wall Street Journal reported. 

Marathon Oil (MRO - Get Report) rose after agreeing to acquire PayRock Energy for $888 million from private-equity firm EnCap Investments. PayRock owns roughly 61,000 net surface acres and current production of about 9,000 net barrels of oil equivalent per day in Oklahoma. The transaction is expected to close in the third quarter.

Walt Disney (DIS - Get Report) climbed 0.6% after its Finding Nemo sequel, Finding Dory, dominated the weekend box office. The film scored $136.2 million in North American ticket sales, securing it the title of highest-grossing animated debut in history.

Walmart (WMT - Get Report) and JD.com (JD - Get Report) announced a strategic alliance on Monday in which the second-largest e-retailer in China will take control of Walmart's Yihaodian direct-sales business. The partnership will give Wal-Mart a better shot at competing in the region's retail industry. JD.com shares climbed 6%.