Why Bernie Sanders is going to keep fighting

President Obama called Sen. Bernie Sanders Sunday afternoon, according to a source familiar with the conversation, reports CBS News' Julianna Goldman.

While the source didn't characterize the conversation, Sanders spokesman told CBS News' Kylie Atwood that the two have spoken on multiple occasions.

The call between the two lasted 30-45 minutes, CBS News' Nancy Cordes reported. Sanders also spoke with the president by phone in mid-May.

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The Vermont senator took the president's call on the side of the road in the financial district in San Francisco, at around 2:30 p.m. ET. Atwood said that as the press bus drove by, Sanders could be seen standing on Market Street, legal pad in hand, as he talked on a cell phone.

Atwood asked Sanders about the conversation Tuesday, but he declined to say much about it.

"I have spoken to President Obama many many times about many issues, and I really think it's not appropriate to talk about my discussions with the president," he told Atwood. "I try to keep that private."

On Tuesday, six states will hold election contests, and 694 delegates are at stake. Sanders' opponent, front-runner Hillary Clinton is expected to surpass the 2,383 number of delegates needed to clinch the Democratic nomination early Tuesday evening. She needs just 23 delegates, according to CBS News' count, to reach 2,383.