Pelosi was one of the last top Democrats to announce their endorsement of Clinton and waited to throw her support behind the former secretary of State until the day of the California primary.

California and five other states on Tuesday are holding voting contests in the Democratic primary between Clinton and Vermont Sen.

The Associated Press on Monday night declared Clinton the presumptive Democratic presidential nominee, saying she had secured pledges from the necessary 2,383 delegates. Sanders's campaign has indicated he hopes to win over superdelegates who have said they'll back Clinton.

"I believe that she will walk into that Oval Office one of the best-prepared people to do so in our country's history; however, it's because she's the best, not because she's a woman," Pelosi said.

She added, however, that it's "pretty exciting" to see the first female presidential nominee for a major political party.

Pelosi also said Sanders "knows better than anyone what's on the line in the election and that we at some point have to unify."

— This report was updated at 7:47 a.m.