On Sunday, U.S. President Barack Obama expressed optimism that an agreement could be reached. "I am confident we can get our fiscal situation dealt with," he said in Bangkok, as he commenced a three-nation Southeast Asia visit.

Kostin was more bullish on the S&P 500 over the medium-term, and forecast the index would end 2013 at 1,575 points, an increase of 16 percent on current levels.

"The S&P 500 has near-term political risk but long-term policy support," he said. "Although we believe investors will have an opportunity over the near-term to buy the S&P 500 at a level below today, portfolio managers with a longer-term horizon should consider increasing equity exposure."

Kostin highlighted that the S&P 500 fell 17 percent between July 25 and Aug. 8 last year, as the U.S. debt ceiling deadline approached. He said those declines had also created an attractive entry point.

"In retrospect, the sell-off created an attractive investment opportunity, given the S&P 500 has since rallied by 21 percent," he said.

On Monday, Michael Crofton, CEO of the Philadelphia Trust Co., told CNBC's "Worldwide Exchange" that U.S. equity investors were waiting for further news on the fiscal deliberations before positioning themselves.

"I do not think we go over the fiscal cliff, but I also do not think we are very close to a solution," he said.

—By CNBC.com's Katy Barnato