"It is money, and only money, that is the reason we are leaving today," Vilsack told reporters at a news conference, later adding, "We have a debt we're going to have to work our way through."
Vilsack, 56, left office in January and traveled to early voting states, but he attracted neither the attention nor the campaign cash of his top-tier rivals - Sen. Hillary Rodham Clinton, Sen. Barack Obama and John Edwards.
"Vilsack's decision reflects the real difficulties these candidates face in mounting viable challenges in a field with names like Hillary Clinton, Barack Obama and John Edwards," said CBSNews.com Senior Political Editor Vaughn Ververs. "That Vilsack was finding little traction in a state he governed for eight years says something about the amount of oxygen being sucked up by those three candidates."
In the most recent financial documents, Vilsack reported raising more than $1.1 million in the last seven weeks of 2006 but only had around $396,000 in the bank. Some campaign finance experts contend candidates will need $20 million by June 2007 to remain viable.
"I came up against something for the first time in my life that hard work and effort couldn't overcome," he said. "I just couldn't work any harder, couldn't give it enough."
Vilsack's campaign had been pegging much of its hopes on staging early wins in his native Iowa and, to a lesser extent, New Hampshire. But he admitted that he would not have been able to raise the money to compete beyond that, particularly in the large number of states expected to hold Feb. 5 nominating contests.
"We could not raise the money that would have allowed us to expand our organization," he said in a conference call with reporters that followed his initial announcement. "With other states threatening to move up the calendar, the premium on money became even more compelling."
He said his decision to drop out had been in the making for the past few weeks, even though he had maintained he was in the race to win as recently as Wednesday at a forum for Democratic candidates in Carson City, Nev.
Joining him at the news conference was his wife, Christie, and his two grown sons.
Vilsack's withdrawal still leaves a crowded field of eight Democrats. He will remain an important figure in the presidential race as former rivals undoubtedly will seek his endorsement and help to win Iowa.
Vilsack, who likely will be considered as a vice presidential nominee, repeatedly declined to endorse another candidate at his news conference.
"A decision now could preserve his chances for a spot on the ticket," Ververs said. "Iowa remains an important state for the general election, and Vilsack's Midwestern appeal would be an attractive quality for the eventual Democratic nominee."
Other campaigns immediately began to seek out Vilsack's well-respected staff, hoping to pick up talented political operatives with experience in the first nominating state.
Clinton issued a statement praising Vilsack and saying, "I have been proud to work with Tom Vilsack for years on the challenges facing our country."
New Mexico Gov. Bill Richardson could benefit from Vilsack's decision, Ververs said, since he is now the only Democratic candidate with executive experience.
Vilsack was the first Democrat to formally enter the 2008 race when he announced his candidacy in November. He conceded at the time that he faced a difficult run.
Trying to counter perceptions that as one of the least known of the prospective candidates he was too much of an underdog to succeed, Vilsack said in a campaign video: "I've never started a race that I've been expected to win, and I've never lost."
As governor of Iowa, Vilsack had carved out a reputation as a centrist balancing his state's budget and refusing to raise taxes, while emphasizing increased spending on such priorities as education, health care and higher wages. Until recently he chaired the Democratic Leadership Council, the party's signature centrist group.
Vilsack initially made the focus of his long shot campaign a plan to end U.S dependence on foreign oil by promoting alternative energy sources.
"Energy security will revitalize rural America, re-establish our moral leadership on global warming and climate security, and eliminate our addiction to foreign oil," Vilsack, a prominent proponent of ethanol, biodiesel and wind power, said at the time.
More recently, Vilsack has been among the more aggressive Democratic candidates in his call to bring U.S. troops home from Iraq, calling for Congress to cut off funding.
Beyond his record as governor, Vilsack tried to sell himself as a candidate with a compelling personal story, which he hoped would spark national interest in his candidacy. He was left as an infant at a Catholic orphanage in Pittsburgh and adopted by what he has described as a "troubled but loving family."
His parents were well-to-do and sent him to a private preparatory school, but his mother was an alcoholic who beat him and his father suffered trying financial reversals.
Vilsack managed to transcend his difficult childhood to build a successful career in law and politics, serving as a mayor, state senator and two terms as Iowa governor.
In a sign that Vilsack might abandon the race, he recently accepted a position lecturing at the Drake University Law School in Des Moines and had become a consultant for MidAmerican Energy Co.