The government shutdown may prove to be a toxic event for the stock market, says Bob Doll, chief equity strategist at Nuveen Asset Management.

Already, the Standard & Poor's 500 Index dropped 0.6 percent Monday amid concern about the shutdown.

"If we don't let this pass soon, we could have looked at the [2013] high for the S&P 500 at 1,729," Doll told CNBC. "I don't want to say that yet. But there's enough noise around this, and the debt ceiling is still in front of us. The market won't like it."

Editor’s Note: Obama’s Budget Takes Aim at Retired Americans

The S&P 500 reached a record 1,729.86 Sept. 19 and since then has dropped 2.8 percent to Monday's close of 1,681.55.

The Senate rejected the House's latest spending bill Monday afternoon, and an agreement was not reached before the midnight deadline for avoiding a government shutdown.

"I think the stock market is saying, 'Hey look, for every week that the government is shut down — not that it'll last that long — about a quarter of 1 percent GDP hit off the fourth quarter,'" Doll said.

Not all market observers are too worried about a shutdown.

"This is not a doomsday situation. At some point someone will blink, and there will be an agreement. . . . I don't think it [the government shutdown] will last more than a few days," Dan Heckman, senior fixed-income strategist at U.S. Bank Wealth Management, told MarketWatch.

"I don’t see a tremendous amount of panic in the markets, and that to me is a hopeful sign that markets sense an agreement, and maybe an agreement that's positive," Heckman added.

Editor’s Note: Obama’s Budget Takes Aim at Retired Americans

Related Stories:

Analysts: Govt Shutdown Is Either a Stock Buying Opportunity or a Disaster

Shutdown Would Cost Economy $300 Million a Day, IHS Says