U.S. debt has climbed to an alarming level, according to DoubleLine CEO Jeffrey Gundlach.

"People are starting to realize that the deficit and debt are totally out of control," Gundlach said on CNBC's "Halftime Report" Tuesday.

Gundlach said the "main reason" the yield curve between 3-year and 5-year Treasury notes is steepening is the ballooning deficit. Last year, U.S. national debt increased by more than 6% of GDP, he said. An even bigger deficit could mean trouble in a recession, said Gundlach, whose DoubleLine has $130 billion in assets under management.

Gundlach — sometimes known as the "bond king" — also flagged trouble in the corporate bond market, which got "dragged down" in the "economic mess that we're in."

"The corporate bond market is so much worse today than it was in 2006," he said.

Among Gundlach's concerns: a corporate bond market that has tripled in size, and a BBB-rated bond market that is now bigger than the junk-bond market. Using leverage ratios alone, "45%, not just of the BBB but the entire corporate bond market would be junk right now," he said, citing figures from Morgan Stanley.

A recession or downturn could "spark" a wave of downgrades from investment grade bonds into junk bonds, he said.

"The economy is in such bad shape to withstand a downturn again," Gundlach said. "The national debt is exploding while we're having some of the best GDP year over year that we've had in recent years."

In the first quarter, U.S. gross domestic product, or GDP, expanded by 3.2%, according to the Bureau of Economic Analysis. That was its best growth to start a year in four years.

Gundlach runs the $50 billion DoubleLine Total Return Bond Fund. Its five-year performance is one of the best in its category, but lagged most of its peers in 2019 with a gain of just 2%, according to Morningstar rankings.

Despite GDP growth and strong employment, if and when a recession does hit, Gundlach said, the U.S. economy is in no shape to handle it without drastic measures.

"The economy's not in any kind of condition for the government to come to the rescue other than really wickedly extraordinary policies a la the [European Central Bank] and the [Bank of Japan]," Gundlach said.