Investors shouldn't rule out the chances of the Federal Reserve rolling out a third round of quantitative easing, as the economy still suffers from fundamental weaknesses and risks recession in 2013, says Jason Trennert, founder and chief investment strategist at Strategas Research Partners.

"It cannot be off the table. I think if you get any sort of sense of weakness, the Fed will be back in there sooner than you think," Trennert tells CNBC.

Under quantitative easing, the Federal Reserve buys assets from banks with the aim of keeping interest rates low and stock prices rising while steering the economy away from a deflationary track.

Editor's Note: You Deserve to Know What Obama and Bernanke Are Hiding From Americans

The Fed has already launched two rounds of quantitative easing, known widely as QE1 and QE2.

QE1 saw the Fed buy $1.7 trillion in assets from banks, mainly mortgage securities, while QE2 saw the central bank snap up $600 billion of Treasury bonds, the latter of which wrapped up on June 30, 2011.

Market predictions for a QE3 have tempered, and the dollar has strengthened on a growing sentiment that the Federal Reserve may allow the economy to heal on its own.

Don't rule it out, says Trennert, as any hints of a weakening economy could have the issue back up front and center.

"I think there's a fundamental reason why the Fed is continuing to be in there, because the structural problems of the economy have not been solved," Trennert says.

The U.S. economy may repeat its 2011 performance, where it started off well and weakened in the second half, Trennert adds.

Plus when fiscal stimulus spending measures dry up and are no longer able to prop up the economy, growth will slow down even further.

"I think there's a decent chance you'll get a mild recession in the U.S. next year simply from fiscal drag," Trennert says.

Political uncertainty can send the economy contracting as well.

"I don't want to offend anyone's politics, but if Barack Obama were to be reelected, that creates a whole other source of issues," Trennert says.

"I think as far as the Bush tax cuts are concerned, especially on dividends and capital gains, that's a worry for me. It's going to be hard to get another round of fiscal stimulus — there are things in the second half of the year that will be somewhat problematic."

Investors have been selling Treasury bonds on waning expectations for an imminent QE3, although they aren't ruling out the possibility altogether.

"There's a reassessment of where the Fed is going, and some long positions are getting liquidated," says Kevin Flanagan, executive director and fixed-income strategist at Morgan Stanley, according to Reuters.

"The market felt heavy in an environment of non-imminent QE3. That doesn't mean they're not going to give you QE3, but the market had priced it in as coming sooner, rather than later."

Editor's Note: You Deserve to Know What Obama and Bernanke Are Hiding From Americans