Politics: U.S. Presidential Campaign Trails (in chronology): Candidates-18

(L-R) Volunteers for U.S. Presidential Hopeful Sen. Barack Obama (D-IL) and his rival candidate, Hillary Clinton (D-NY) place signs outside the Iowa Brown & Black Presidential Forum in North High School in Des Moines, Iowa, on Dec. 1, 2007. The forum gathered opposing democratic candidates with a month left until the Iowa caucus. (For Getty Images)

(L-R) Volunteers for U.S. Presidential Hopeful Sen. Barack Obama (D-IL) and his rival candidate, Hillary Clinton (D-NY) place signs outside the Iowa Brown & Black Presidential Forum in North High School in Des Moines, Iowa, on Dec. 1, 2007. The forum gathered opposing democratic candidates with a month left until the Iowa caucus.  

(For Getty Images)