Democrat Tom Vilsack is abandoning his bid for the presidency after struggling against better-known, better-financed rivals, citing money -- the so-called "invisible primary" -- as the only reason for getting out of the race.
Vilsack left office in January and traveled through states holding early tests of strength. He had faced a tough challenge from rivals such as New York Sen. Hillary Rodham Clinton, Illinois Sen. Barack Obama and John Edwards, who have had more success raising money and attracting attention -- even in Vilsack's home state of Iowa.
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 race.
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.