A senior campaign adviser to Sen. Bernie Sanders Bernie SandersSirota reacts to report of harassment, doxing by Harris supporters Republicans not immune to the malady that hobbled Democrats The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election MORE said Monday that the Vermont independent is “angry” because he feels like voters aren’t being heard.

“He is a little bit angry,” Chuck Rocha told Hill.TV. “He’s angry because he feels like there's a lot of frustration out there with America.”

“People come up to him and tell him things that make him very angry because it’s like nobody is listening to them about whatever issue it may be,” Rocha added, citing immigration as an example.

Sanders’s campaign has expressed frustration with the way the candidate has been portrayed in the media, at times accusing the press of being biased against him.

During an interview with Hill.TV last week, Sanders spokeswoman Briahna Joy Gray doubled down on those criticisms.

“There is sort of a thinly veiled contempt for the notion that Bernie Sanders has been saying the same thing for 30 years,” she said. "But when you talk to the people who support Sen. Sanders, people who are kind of new to him even and hear him at his rallies, what they say is, 'Wow, it's really heartening to hear someone is right about these issues 30 years ago.' "

Sanders has regularly placed second in national polls behind former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE, the Democratic front-runner.

—Tess Bonn