Democratic Senator Hillary Clinton has taken the first step towards running for the US presidency in 2008.
Sen Clinton, 59, wife of former President Bill Clinton, announced on her website: "I'm in to win."
The former first lady has set up a presidential exploratory committee, testing the waters for a full bid.
Her announcement comes days after African-American Democratic Senator Barack Obama said he had formed an exploratory committee.
In a videotaped and written message on her website, Sen Clinton said it was time "to overcome six years of Bush administration failures.
"The stakes will be high when America chooses a new president in 2008," she said.
"Only a new president can regain America's position as a respected leader in the world.
"This is a big election with some very big questions. How do we bring the war in Iraq to the right end?" she asked, in addition to mentioning several domestic issues, including health care, the environment and "energy independence".
Sen Clinton said she planned to answer questions in web chats on three consecutive nights, starting on Monday.
Her announcement brings to at least seven the number of declared Democratic presidential hopefuls.
The BBC's Matt Frei in Washington says Sen Clinton's announcement has not surprised many but the specific timing of it this week is significant.
He says the senator was thought to want to declare her intentions later in the year, but her hand was forced by the man who is currently capturing the headlines and many of the hearts of the democratic rank and file - Mr Obama.
Sen Clinton would become the first former first lady to seek presidential office, while Mr Obama's entry into the race would raise the possibility of the US having its first black president.
Mr Obama is one of the Democratic Party's rising stars, having electrified the 2004 Democratic convention with a powerful speech.
Born 26 October, 1947 in Chicago
Attended Wellesley College
Graduated from Yale Law School in 1973
Married Bill Clinton in 1975
Campaigner for expanding health insurance coverage and woman's rights
Elected New York senator in 2000; re-elected 2006
Sen Clinton is currently serving as a senator for New York, having won a second term by a landslide last November.
Correspondents say that her unbeatable name recognition and unmatched fundraising ability make her a clear front runner for the Democrats.
Yet she is also seen as a divisive figure. Some estimates say one in three Americans would never vote for her.
Our correspondent says Sen Clinton's biggest policy problem is that she supported the Iraq war and at one stage even called for more troops to be deployed there.
Mr Obama's biggest problem is that as a 45-year-old first-term senator in a time of war, he is relatively inexperienced, our correspondent adds.
Meanwhile, Republican senator Sam Brownback on Saturday declared his intention to seek his party's nomination for a 2008 run.
Correspondents say he may get strong support from social conservatives and the religious right, but he faces as many as 10 other potential Republican candidates.