In a videotaped message posted on her Web site, Clinton said she was eager to start a dialogue with voters about challenges she hoped to tackle as president — affordable health care, deficit reduction and bringing the "right" end to the Iraq war.
"I'm not just starting a campaign, though, I'm beginning a conversation with you, with America," she said. "Let's talk. Let's chat. The conversation in Washington has been just a little one-sided lately, don't you think?"
Clinton's announcement, while widely anticipated, was nonetheless historic in a fast-developing campaign that has already seen the, Democratic Sen. Barack Obama of Illinois.
In an instant, Clinton became the most credible female candidate ever to seek the presidency and the first presidential spouse to attempt to return to the White House in her own right. Her husband, Bill, served two terms as president from 1993 to 2001.
"I am one of the millions of women who have waited all their lives to see the first woman sworn in as president of the United States — and now we have our best opportunity to see that dream fulfilled," said Ellen Malcolm, president of EMILY's list, which raises money for Democratic women who run for office.
With her immense star power, vast network of supporters and donors and seasoned team of political advisers, the 59-year-old Clinton long has topped every national poll of potential Democratic contenders.
But since joining the field, Obama has secured the backing of a number of prominent fundraisers, including billionaire philanthropist George Soros, stepping up the pressure on Clinton to disclose her plans.
Obama said in a statement soon after Clinton's entry, "I welcome and all the candidates, not as competitors, but as allies in the work of getting our country back on track."
Last week – after Obama's announcement but before her own – Sen. Clinton told CBS News' Harry Smith that she was glad to have Obama in the race.
"It's terrific that we're going to have a very vigorous primary, on both sides," Clinton said. "I'm looking forward to a spirited and substantive debate about issues, about goals, about aspirations, about experience, about the kinds of things voters would be interested in."
Her controversial tenure as first lady left her a deeply polarizing figure among voters, leading many Democrats to doubt Clinton's viability in a general election.
But Clinton's top political advisors say they've heard it all before, reports CBS News correspondent Joie Chen.
"These are a lot of the same questions we faced in 1999, when she decided to run in a state she never lived in: Could she do it? Would she do it?" says Clinton senior advisor Howard Worlfson. "This is a woman who is no stranger to hard work. She's going to work her heart out and earn every vote."
In a detailed statement posted on her Web site, Clinton sought to acknowledge and bat away such doubts.
"I have never been afraid to stand up for what I believe in or to face down the Republican machine," she wrote. "After nearly $70 million spent against my campaigns in New York and two landslide wins, I can say I know how Washington Republicans think, how they operate and how to beat them."
Recently, Clinton has clashed with many in her own party over the Iraq war.
Clinton supported the 2002 resolution authorizing military intervention in Iraq. She has refused to recant her vote or call for a deadline for the removal of troops. She has announced her opposition to President Bush's troop increase in Iraq and has introduced legislation capping troop levels.
"A woman candidate could find it easier to run in peacetime, rather than wartime, but Senator Clinton's tried to position herself as a serious person on national security," said Andrew Polsky, a presidential historian at Hunter College. "But that means she's staked out difficult position on the war that won't make it easy for her to get the Democratic nomination."
With a $14 million campaign treasury, Clinton starts with an impressive fundraising advantage over the rest of the Democratic field. But Obama and others have started to secure fundraising commitments from New York, California and other deep-pocketed, Clinton-friendly areas.
Her creation of a presidential exploratory committee, announced Saturday, allows her to raise money for the campaign; she already has lined up campaign staff.
In tone and substance, Clinton's videotaped announcement recalled her first Senate race in New York in 2000, where she conducted a "listening tour" of the state's 62 counties before formally entering the contest.
She promised a three-day series of Web chats with voters beginning Monday and prepared a campaign swing late this coming week through the early voting state of Iowa, while a visit to New Hampshire was in the works.
On Sunday, New Mexico Gov. Bill Richardson was also set to enter the Democratic field; if elected, he would be the first Hispanic president.
For the short term at least, the outsized candidacies of Clinton and Obama were expected to soak up the lion's share of attention.
Obama, who launched his own presidential committee on Tuesday, praised Clinton as a friend and colleague.
"I welcome her and all the candidates, not as competitors, but as allies in the work of getting our country back on track," he said in a statement.
However, there are a lot of people who believe that Obama and Clinton could cancel each other out and make room for a second tier candidate such as John Edwards or Joe Biden, reports CBS News correspondent Gloria Borger.
Campaigning in New Hampshire, Connecticut Sen. Chris Dodd did not comment specifically on Clinton's announcement, but said: "I'm not one for exploratory committees. You're in or you're not."
Other Democratic contenders include former Iowa Gov. Tom Vilsack; Ohio Rep. Dennis Kucinich and former North Carolina Sen. John Edwards, the party's 2004 vice-presidential nominee. Delaware Sen. Joe Biden has said he will run and planned to formalize his intentions soon. Massachusetts Sen. John Kerry, the party's 2004 standard bearer, is also contemplating another run.
An influential player in her husband's political career in Arkansas, Hillary Clinton leapt to the national scene during the 1992 presidential campaign when husband and wife fought to survive the scandal over Gennifer Flowers' allegations of a lengthy affair with Bill Clinton when he was the state's governor.
The Clintons appeared together on 60 Minutes to talk about their marriage — Hillary Clinton's first famous "" moment.
As first lady, Clinton headed up a disastrous first-term effort to overhaul the health care insurance system. There was more controversy as the couple battled allegations of impropriety over land deals and fundraising, missing records from her former Arkansas law firm and even her quick and hefty profits from an investment in cattle futures.
There was no letup in the second term. The president found himself denying — then admitting — having a sexual relationship with White House intern Monica Lewinsky. As he battled impeachment and possible removal from office, his wife's poll numbers rose.
Her political career began to take shape in late 1998 when New York Democrat Daniel Patrick Moynihan announced he would not seek re-election to the Senate seat he had held since 1976.
The campaign trail was not always friendly. For almost every cheer, there was a shouted "Go home, Hillary!" and the emerging Republican theme that carpetbagger Clinton simply wanted to use New York as a launching pad for a later presidential run.