Clinton posted a video on her official website in which she said, "I'm in, and I'm in to win." She announced plans to establish an exploratory committee that will raise money for her campaign effort.
"I announce today that I'm forming a presidential exploratory committee. I'm not just starting a campaign, though, I'm beginning a conversation -- with you, with America," she said. "Because we all need to be part of the discussion, if we're all going to be part of the solution -- and all of us have to be part of the solution. Let's talk about how to bring the right end to the war in Iraq, and to restore respect for America around the world."
Clinton, 59, was the country's first lady when her husband, Bill Clinton, was U.S. president from 1992 to 2000. Now she represents the state of New York in the U.S. Senate.
Clinton's announcement comes just days after another Democratic senator, Barack Obama, shook up the contest by announcing that he is also forming a campaign committee. If successful, Obama would be the country's first black president.
Clinton is considered the front-runner, with Obama and 2004 vice presidential nominee John Edwards top contenders.
(compiled from agency reports, hillaryclinton.com)