The vice president -- and his wife, Jill Biden -- voted early while at their home in Wilmington, Delaware on Friday, the vice president's office confirmed to CNN.
While Biden has not endorsed either Hillary Clinton or Bernie Sanders for the Democratic nominee, he has not been shy about talking about the state of the race and the two candidates.
Just this week in an interview, Biden praised Sanders
for thinking big about how to lead the country, seeming to dismiss the Vermont independent senator's critics who say his proposals are not realistic.
"I like the idea of saying, 'We can do much more,' because we can," Biden told The New York Times in an interview published Thursday.
"I don't think any Democrat's ever won saying, 'We can't think that big — we ought to really downsize here because it's not realistic,'" Biden said. "C'mon man, this is the Democratic Party! I'm not part of the party that says, 'Well, we can't do it.'"
It's not the first time Biden has made comments interpreted as support for the Vermont senator. During a CNN interview with Gloria Borger in January, Biden backed Sanders' fight on income inequality and described Clinton as "relatively new"
to the movement.
Biden mulled whether to run for the Democratic nomination last year, before he put an end to speculation in an October announcement that he wouldn't seek the presidency.
The vice president, who has wanted to reach the highest office for decades, said he regrets "every day" that he didn't enter the 2016 presidential race -- but that his decision was in his family's best interests.
The Delaware primary, scheduled for Tuesday, is a closed primary. Pennsylvania, Rhode Island, Connecticut and Maryland also have their primaries that day.