The bulls may be treading water.

According to Ned Davis Research's Ed Clissold, there's a high probability a record year-end rally will give way to a painful 2019.

"You could be looking at the first 20 percent-plus decline in the S&P since the financial crisis," the firm's chief U.S. strategist said Tuesday on CNBC's "Futures Now."

His worst-case scenario is a 30 percent plunge next year.

"Our primary list of concerns is on the earnings front," Clissold said. "Earnings growth north of 20 percent isn't sustainable, especially when you're nine years into an economic expansion."

Clissold, a secular bull, isn't calling for a major, drawn-out recession. Nevertheless, he said he's on bear market watch due to warning signs indicating a tired bull market.

"We're certainly a lot further along than we were a couple of years ago certainly in the economic cycle," Clissold said. "From a technical standpoint, leadership has certainly gone from being more aggressive to more defensive. ... More defensive sectors like health care, utilities, staples and telecom have been leading as of late."

Despite his cautiousness, Clissold hasn't officially downgraded his positive outlook on U.S. stocks. His year-end price target is 2,900 — about a 1.5 percent gain from current levels. He said stocks could soon see a short-lived pullback mainly do to seasonal trends at any time, and then grab gains as the year winds down.

"From [the] 2009 lows, whenever we got a pause, we'd break out of what we'd call a breadth thrust. You'd get a huge percentage of stocks moving up together, and that's a sign of a very healthy market," Clissold said. "We haven't gotten one since the February correction, and that's a change in character. It doesn't mean the market can't rally for a while. But, that's what happens at the end of a bull market, not at a beginning for a bull market."