Warner, a centrist, had been one of the more aggressive and successful early campaigners among the large group of Democrats pursuing possible presidential bids. He had lined up considerable financial support, made 67 trips to 28 states and hired a large number of advisers with national campaign experience.
There had been speculation that he would be the first candidate to formally announce a 2008 candidacy. But in a statement posted on his Web site and, later, at a hastily called news conference, Warner said that for personal reasons, it was not "the right time for me to take the plunge. At this point, I want to have a real life."
The move by Warner, a potentially strong contender, could help New York Sen. Hillary Rodham Clinton, the early favorite for the Democratic nomination.
It also could assist others who have been maneuvering to make themselves the leading alternative to Clinton. These include Sen. Evan Bayh of Indiana, whose message - that he is a Democrat who has won in a Republican state - is similar to Warner's, and former Sen. John Edwards of North Carolina, the party's 2004 vice-presidential nominee and now the lone Southerner in the likely '08 field.
"It's a huge boost to Evan Bayh," said Kenneth Baer, a speechwriting and policy consultant to Warner's organization. "I think it puts pressure on Edwards, because now he'll have to win in South Carolina," the first Southern primary state.
Warner's decision, and its timing, shocked those who have known the former governor for years and were convinced that he had decided to run. His announcement came in the final weeks of a closely fought national election campaign, in which Warner has played a very active role.
Rep. Benjamin L. Cardin of Maryland, a Democratic Senate candidate, was one of many taken by surprise by news of Warner's move. Cardin said Warner had agreed to come to Maryland to campaign for him and that he wondered whether the visit would still take place.
Warner, a 51-year-old venture capitalist, said he plans to go ahead with his campaign schedule, including a trip late yesterday to Iowa, where the presidential process starts, for appearances on behalf of Democratic candidates.
"I hope they're still excited to see me," he said, smiling.
Barred by law from seeking re-election as governor, Warner has traveled extensively since leaving office in January. By his count, he was a major draw at 86 campaign events in 25 states and helped raise, or made direct donations, of $7.3 million to other Democrats. His political action committee has collected more than $9 million - and given more to other Democrats than the PACs of other potential presidential candidates, he said.
"This is not a decision based on a choice about whether I would win or lose," Warner said. "I can say with complete conviction that, 15 months out from the first nomination contests, I feel we would have had as good a shot as any of the potential candidates in the field."
But he said that after wrestling for weeks with the decision, and "a lot of reflection, prayer and soul-searching," he had concluded that his family comes first.
Privately, friends said that Warner's wife, Lisa, and eldest daughter, Madison, 16, were opposed to a presidential run, though Warner denied that in remarks to reporters, joking that his wife's position had moved from "negative to neutral."
Warner said he still wants to "serve - whether in elective office or some other way." He did not rule out another try for governor in 2009.
The decision also keeps his name alive as a potential 2008 vice-presidential nominee. In 1991, Al Gore announced that he would not be a candidate for president, citing a desire to spend more time with his family, which had been shaken by his young son's near-fatal accident while leaving a Baltimore Orioles game. Less than a year later, Gore returned to the national scene as Bill Clinton's running mate.
Warner left open the possibility of a future presidential bid, but he acknowledged that "things will probably never be as aligned as they are right now."