Sen. John Kerry, the 2004 Democratic presidential nominee, announced Wednesday that he won't make a run for the White House in 2008.
"I’ve concluded this isn’t the time for me to mount a presidential campaign," Kerry said on the Senate floor. "It is the time to put my energy to work as part of the majority in the Senate to do all I can to end this war and strengthen our security and our ability to fight the real War on Terror."
Kerry made the announcement after an extensive speech on Iraq in which he recited the history of violence between Shiites and Sunnis and called on his congressional colleagues to help stabilize the war-torn country and withdraw U.S. troops.
"I intend to devote all my efforts and energies over the coming two years not to another race for the presidency for myself but to doing whatever I can to ensure that the next president can take the oath with a reasonable prospect of success for him or her for the United States," Kerry said.
The Massachusetts senator also was taping a video message to e-mail supporters to thank them for their support.
"Thanks to you, we have a new Democratic Congress that is fighting to stop the administration's disastrous course in Iraq, thanks to you we can be a Congress that addresses issues like climate change and health care, and thanks to you, change is coming to Washington," Kerry said in a written e-mail to backers.
Senate Majority Leader Harry Reid, D-Nev., took to the Senate floor after Kerry's announcement to thank him for his work in the Senate.
"He is one of those people that has meant so much to me. And being a role model for the things that I do and for the things that I think the American people should focus on," Reid said.
Reid also spoke about Kerry's failed run for president in 2004.
"John Kerry came within a few votes to be the president of the United States in one of the dirtiest, most negative, unfair campaigns I have ever witnessed," Reid said.
Sen. Edward Kennedy, D-Mass., a strong supporter of Kerry's 2004 presidential campaign, said Kerry has "been a true hero in every sense of the word."
"I say to the people of the country how proud we are of John Kerry. I think all of us in Massachusetts know what an outstanding senator he has been for our state and for our country," Kennedy said.
Kerry decided "he could accomplish more for the country in the Senate than as a presidential candidate," a source close to Kerry told FOX News. The four-term senator is up for re-election in 2008 and will run for office again, according to a Democratic official who spoke on condition of anonymity.
The source said the senator chose not to enter the crowded presidential field so that he can re-dedicate himself to getting the United States out of Iraq.
"Iraq, Iraq, Iraq. Senator Kerry believes he can do more to get the country out of Iraq in the Senate than he can as a presidential candidate," the source said. "He can do more by working in the Senate and traveling to the Middle East to get us out of Iraq than he can by campaigning in Cedar Rapids, Iowa."
Kerry, 64, criticized President Bush Tuesday night after the State of the Union address, saying, "The president missed a golden opportunity tonight to admit that he made a mistake in Iraq and to share with the American people a plan for gradually removing our troops."
Kerry made his announcement during a pause in Senate debate on raising the minimum wage. To do so, he sought time from Kennedy, the manager of the minimum wage bill.
When Kerry hedged on the 2008 race earlier this year, Kennedy suggested he might not support Kerry this time around. He later said that if Kerry decides to join the race, he will back him.
Many other of Kerry's supporters had decided not to wait for his decision, and bolted to other campaigns. On Tuesday, Sen. Hillary Clinton, who over the weekend made her intention to run known, won the support of top Democratic donor Hassan Nemazee, Kerry's New York finance chair in 2004 and a key force behind the success of the Democratic Senatorial Campaign Committee in 2006.
With $12.5 million still in his campaign account, Kerry will have notable impact on the 2008 Democratic race.
Kerry leaves a growing field of Democrats running or likely to run including former vice presidential running mate and Sen. John Edwards of North Carolina. Other contenders include Mike Gravel of Alaska, Rep. Dennis Kucinich of Ohio, Mexico Gov. Bill Richardson and former Gov. Tom Vilsack of Iowa, Sens. Barack Obama of Illinois, Joe Biden of Delaware and Christopher Dodd of Connecticut and Clinton.
Kerry lost the White House in November 2004 when the swing state of Ohio went to Bush. His campaign was roundly criticized for failing to make a forceful enough response to Republican criticism as well as charges by conservative groups that he did not deserve the medals he won for combat in the Vietnam War.
He also botched an explanation of his vote to pay for troops in Iraq by rolling back some of Bush's tax cuts. The phrase "I actually did vote for the $87 billion before I voted against it" was repeated over and over again in campaign advertising against the senator.
Earning the reputation for flubbing execution of his thoughts, in October Kerry earned derision when "botched" a joke that was trying to suggest Bush was stupid but instead ended up insulting the entire U.S. Armed Services.
Kerry apologized, then hastily scrapped several days of campaigning for fellow Democrats as party leaders urged him to avoid becoming an unwanted issue in a campaign they were on the way to winning.
The Massachusetts lawmaker decided to clarify his political plans on a day in which he participated in a debate over the war in Iraq by invoking memories of Vietnam. At the committee hearing, he said a memorable question he first posed in 1971 had relevance today: "How do you ask a man to be the last person to die for a mistake?"
Despite his difficulties on a national level, Kerry customarily rolls up large victory margins at home in Massachusetts. He won his first term in 1984.
FOX News' Major Garrett and The Associated Press contributed to this report.