The relatively low-key announcement is a recognition that the former secretary of state and wife of former President Bill Clinton, viewed as the heavy favourite for the nomination, already has huge name recognition and needs an uncomplicated start to the long campaign ahead.
Following the announcement, Clinton will travel to the key early voting states of Iowa and New Hampshire, said the source, who asked to remain unidentified.
"She's expected to make her intention to run known on Sunday," the source said.
A representative for Clinton, who also served as a senator from New York, declined to comment.
Once she starts traveling, Clinton is expected to participate in small events to get up-close to voters to hear their concerns and explain why she wants to be president.
This is different from the campaign rollouts of Republicans Ted Cruz and Rand Paul, who both held big events to announce their 2016 campaigns.
Democratic strategist Steve Elmendorf said a more modest rollout would be helpful for Clinton, who lost the 2008 Democratic presidential race to Barack Obama, now serving his second term.
"She hasn't been out campaigning for a long time," he said. "You need to get your sea legs back. All candidates benefit from listening to people and getting a sense of what concerns voters have and respond to them. It's a long campaign. There's no need to start out with a bunch of big rallies."
Clinton appears likely to face limited competition for the Democratic nomination. Former Virginia Senator Jim Webb and former Maryland Governor Martin O'Malley are among the few Democrats to take steps toward a bid so far.
Clinton's reliance on social media amounts to the adoption of a tactic that Obama used in 2008 to draw attention to his candidacy and raise huge sums through small donations.
Her use of social media is also an appeal to young voters, whom Clinton needs to become American's first woman president. In 2012, young political staffers made the now 67-year-old Clinton an Internet sensation with the blog "Texts from Hillary."
Also on Friday, Clinton released an update to her memoir, "Hard Choices," in which she described her final days as secretary of state and her feelings about her first grandchild, as her campaign is expected to concentrate on making her relatable to ordinary Americans.
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