NEW YORK (MarketWatch) -- The U.S. economy has a "significant likelihood" of entering a double-dip recession if the government doesn't step in to help the unemployed, economist Robert Shiller told MarketWatch News Break on Wednesday.

The Yale University professor and author of the best-selling book "Irrational Exuberance" pinned the probability of a double-dip recession at more than a 50-50.

Shiller pointed to the nation's stubbornly-high unemployment as a root cause of lingering economic woes. And with the Federal Reserve running out of bullets to fight a second recession, he urged Congress to join the battle and focus on putting people back to work.

"Beyond the Fed, I'd like to see the government take a renewed stimulus package focused on creating jobs [and] on activities that involve a lot of people," Shiller said. Listen to Shiller interview.

The Fed released a statement on Tuesday showing it took a step to spur economic growth while keeping interest rates at record lows. That did little to encourage investors. In recent trading, the Dow Jones Industrial Average DJIA, +1.19% was down 242 points, or 2.2%, at 10,401 points. The S&P 500 Index SPX, +0.82% was down 2.7%.

While the Fed still has other ways to support the economy, such as halting interest payments on reserves or even taxing reserves, that alone may not be enough to prevent risks of a double-dip.

"The Fed's latest statement shows they're on this, but I'm not so sure Congress is on this," said Shiller. "There is significant likelihood of [a second recession] if the government doesn't do something. I'm worried [unemployment] is not going to self-correct."

In response to having to dig even deeper into fiscal debt, Shiller said costs of a jobs-oriented stimulus should not be an overriding concern.

"If we focus on creating job, it's not as expensive as you might think."