With the focus of the 2016 presidential campaign shifting to issues of national security, following this month's San Bernardino terror attack, Bernie Sanders -- whose campaign focuses on economic issues -- is finding it harder and harder to keep up with Democratic front-runner Hillary Clinton.

The Vermont senator's conundrum was on display during Saturday's Democratic presidential debate, which largely focused on security and foreign policy issue that play to Clinton's strengths, The New York Times reported.

Sanders used the event to point to his rival's relationships to Wall Street executives and the controversies surrounding Clinton's four-year tenure at the State Department, but later had to apologize for a breach by his campaign of her voter data and even complimented her on being a transformative first lady.

Clinton, for her part, portrayed herself as an experienced leader and insisted that she was the best candidate to make critical security decisions in the Oval Office.

Her challenger's attempt to pivot back to economic issues, meanwhile, seemed desperate at times. Sanders conceded that Americans were "anxious" about the rise of the ISIS jihadist group and the recent attacks in Paris and California, but insisted that other concerns should be just as important.

"They're [also] anxious about the fact that they are working incredibly long hours, they're worried about their kids, and they're seeing all the new income and wealth, virtually all of it, going to the top 1 percent," the socialist senator said. "And they're looking around them, and they're looking at Washington and they're saying, 'The rich are getting much richer. I'm getting poorer. What are you going to do about it?'"

Sanders' national poll numbers, meanwhile, still resemble those of then Sen. Barack Obama when he ran against Clinton eight years ago, the Dallas Morning News noted. But some experts have already written off the Vermont senator's campaign, according to The New York Times.

"Hillary is now in the driver's seat in a way she has never been before," Arnie Arnesen, a New Hampshire talk radio host who has expressed admiration for Sanders, told the newspaper. "It's Bernie's turn to pivot, [and] I don't know if he can."