Stocks could test new lows as soon as 48 hours after the Federal Reserve announces the first interest rate hike in more than nine years, Strategas Research Partners' Jason Trennert said Monday.
The chief investment strategist told CNBC's "Squawk Box" he would be an "aggressive buyer" on any such dip. The move would clear up uncertainty in the market, said Trennert, and would likely be accompanied by dovish language from Fed Chair Janet Yellen that would assure markets she won't derail the economic recovery, sending stocks higher.
The Federal Open Market Committee will announce its decision on its benchmark fed funds rate Thursday. The central bank has held that rate near zero since 2008.
It will be hard to make new highs until the market gets clarity on interest rates, Trennert said. Afterward, traders can focus on strong economic fundamentals.
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"The U.S. economy, if it can't withstand a 25 basis point increase in interest rates, we have other problems and we should look at other policies—fiscal, regulatory, trade—to help us out," he said.
Philip Orlando, chief equity strategist at Federated Investors, said he too expects a sharp downside move in stock markets after liftoff. But there is also risk to holding off on the hike, he added.
"The raise in interest rates typically has a knee jerk reaction down," he told "Squawk Box." "If the Fed doesn't [increase rates], the thought process is what do they know that we don't know in terms of economic growth or earnings growth?"
However, he too believes the U.S. economy is capable of withstanding the normalization of rates.
"Over the course of the last 60 years or so, a normal fed funds rate is 5 percent. We're talking about getting the funds rate to 1 percent maybe by the middle of next year, and Yellen stopping at that point," he said.
"That 1 percent, given the complexity and size of the U.S. economy, is not going to derail economic growth here, corporate earnings, or ... market performance."
The S&P 500 will possibly retest its 52-week low of 1,820 after a rate hike, but will then head higher, he said. "We get this behind us and we start to focus on a resumption of economic growth, resumption of earnings growth."