A top adviser to the former first lady and secretary of state made the 2016 plans official with an email message Sunday to veterans of her first presidential campaign.
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Clinton is aiming to be the first female president.
Clinton enters the Democratic primaries in a strong position to succeed her rival from the 2008 campaign, President Barack Obama.
Following Sunday's announcement, she's expected to travel soon to early voting states, including Iowa and New Hampshire.
Clinton has signaled that she intends to run on strengthening economic security for the middle class and expanding opportunities for working families.
Republicans are already trying to link Clinton to Obama. They say electing her would be no different from giving Obama a third term.
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