Two sources close to the Clinton campaign say former Secretary of State Hillary Clinton will announce her candidacy for president of the United States "as early as Sunday."

She is expected to make the announcement on social media, with campaign stops planned for next week, including to first-in-the-nation caucus state Iowa.

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On Friday, Clinton also published a new epilogue to her book "Hard Choices" on The Huffington Post. In the text, she hinted that the birth of her granddaughter Charlotte was a moment that reinforced her future plans. "Becoming a grandmother has made me think deeply about the responsibility we all share as stewards of the world we inherit and will one day pass on. Rather than make me want to slow down, it has spurred me to speed up," she writes.

Clinton, who ran unsuccessfully for the Democratic nomination in 2008, now enters the race as the undisputed frontrunner for her party’s nod. Only a handful of other Democrats have expressed interest in pursuing a White House bid, including former Maryland Gov. Martin O’Malley, former Virginia Sen. Jim Webb and independent Sen. Bernie Sanders of Vermont. But none have Clinton’s broad name recognition, massive fundraising apparatus and sky-high approval ratings within the party.

If she captures the Democratic Party nomination, Clinton will be the first woman in American history to top a general election ticket for a major political party.

While Clinton’s nascent campaign has been building for months, the run up to her announcement has not been without controversy. Clinton has struggled to explain why she used a personal email account and private server during her tenure at the State Department. The Clinton Foundation, founded by former President Bill Clinton after his eight years in the White House, has acknowledged that it failed to properly disclose donations from foreign governments. And Republicans on Capitol Hill continue to accuse her of a lack of transparency about her response to the 2012 attacks on U.S. facilities in Benghazi, Libya.

Still, Clinton remains relatively popular, particularly with Democrats. In a March 2015 NBC/WSJ poll, Clinton got a positive rating from 44 percent of Americans, while 36 percent gave her a negative rating. In the same poll, 86 percent of Democrats said they could see themselves supporting Clinton for the Democratic nomination.

— Kristen Welker, Andrea Mitchell and Carrie Dann