Joe Biden may win the Democratic nomination, but he cannot beat President Donald Trump, '"Bond King" Jeffrey Gundlach told CNBC on Thursday.

"I think Joe Biden is completely unelectable," the DoubleLine CEO said on "Fast Money Halftime Report."

Gundlach has repeatedly warned about the threat Sen. Bernie Sanders poses to financial markets, suggesting during last week's historic sell-off that Sanders' ascendancy played a key role in the turmoil.

But the state of the Democratic primary has changed dramatically since Tuesday, when Biden won 10 states and regained his status as the front-runner for the nomination.

The same reason why Sanders underperformed expectations on Tuesday is the same reason why Biden would not be able to defeat Trump in a general election, Gundlach predicted.

"[Sanders] unfortunately appeals to voters that don't vote. He appeals to low-turnout segment of the population, young people," Gundlach said. "And the younger people just didn't show up on Super Tuesday."

Younger voters tend to favor the Democratic Party, although they typically do not vote as frequently as older Americans. In 2016, Hillary Clinton won 55% support from voters ages 18 to 29 to Trump's 37 percent. With Biden as his vice president, Barack Obama earned 60% of the youth vote in 2012.

But if Sanders struggled to bring in younger voters, Biden probably will, too, Gundlach said.