Hedge fund manager Cooperman says it's time for the bull market 'to rest'

• Omega Advisors' chairman predicts the market will be "trendless/uninteresting at around current levels for a number of months."
• Principal risk to the bull market is Trump and Republican-led Congress failing to come together, Cooperman says.
• Omega's main fund was up 6.5 percent last quarter, barely more than the market.

Leon Cooperman, chairman and CEO of Omega Advisors, expects the market to be flat through the summer due to uncertainty over President Donald Trump's economic agenda.

"It is time for the U.S. equity market to rest," Cooperman wrote in the April 7 letter to clients.

As a result of the domestic political uncertainty, geopolitics and market valuation, the hedge fund manager expects the U.S. stock market will be "trendless/uninteresting at around current levels for a number of months," he wrote.

Cooperman then predicts the bull market will resume if Washington doesn't get in the way.

"Our core views remain that the in-place U.S. equity bull market should persist for quite a while longer and that U.S. fixed income securities are unattractive," stated the letter. "As we will see, a principal risk to our expectation for a pleasant U.S. share landscape is a political one, an inability of President Trump and the Republican Congress to deliver on their pro-growth economic agenda."

His fund barely edged past the stock market last quarter, according to the letter. The Omega Capital Partners LP fund generated a 6.5 percent first-quarter net return versus the S&P 500 total return of 6.1 percent.

Cooperman founded Omega Advisors in 1991. The firm has approximately $3.6 billion assets under management currently, according to its website. That's way down from their peak after the SEC accused Cooperman of insider trading last year, allegations that he said as recently as last week that are without merit.

The market will "lift moderately, likely to new highs" after August and September as investors gets more "clarity" on what parts of President Trump's economic agenda will be implemented, the note states.

"The reflation trade has stalled," he wrote. "This odd combination of an equity market lifting on better growth expectations while risk-off asset classes and sectors outperform may possibly suggest less investor confidence in the U.S. economic and earnings growth outlook and economic policy."

— CNBC's Scott Wapner and Patricia Martell contributed to this story.