The stock market advance that began in March 2009 may still have a ways to go, says Tom Lee, chief U.S. equity strategist for JPMorgan Chase.

"This could be only the middle innings of what could be one of the longest bull markets in history," he tells CNBC. "There is a lot of firepower to fuel this rally. There is a lot of cash on the sidelines, consumers have delevered."

The Standard & Poor's 500 Index has gained 169 percent from its March 9, 2009, low, though it has slid 1.8 percent so far this year.

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Lee said he's still "very optimistic this is a double-digit year for the S&P." He sees the index ending the year at 2,075, up 14 percent from Tuesday's close.

Market sentiment changed last week when stocks rebounded from Monday's sharp decline, he explains. "All of a sudden . . . we got the 'long onlys' coming back in and the hedge funds." Long onlys refers to fund managers who only go long stocks, not short.

This year is looking somewhat like last year, when about half the stock market struggled "until March and May," Lee notes.

Jim Cramer, host of CNBC's "Mad Money," is less enthusiastic. He fears that weak employment statistics could hurt the stock market. Non-farm payrolls rose only 113,000 in January.

"Employment growth is the most important thing for profits, and the profits have to keep going up" for stocks to keep rising, Cramer tells Yahoo.

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