Vice President Biden defends the administration's foreign policy --€” and questions Mitt Romney's ideas — on Thursday at New York University. Lucas Jackson/Reuters/Landov hide caption

itoggle caption Lucas Jackson/Reuters/Landov

Republican presidential candidate and former Massachusetts Gov. Mitt Romney speaks at a campaign rally in Tempe, Ariz., on Friday. Jae C. Hong/AP hide caption

itoggle caption Jae C. Hong/AP

Courting Tea Party voters in Philadelphia on Monday night, Mitt Romney speaks next to a large statue of Benjamin Franklin at the Franklin Institute. Tim Shaffer/Reuters/Landov hide caption

itoggle caption Tim Shaffer/Reuters/Landov

Rick Santorum won the Louisiana Republican presidential primary Saturday, beating front-runner Mitt Romney in the race to challenge President Obama. Jae C. Hong/AP hide caption

itoggle caption Jae C. Hong/AP

A screen shot of President Obama from the trailer for his campaign's movie The Road We've Traveled. BarackObama.com/YouTube hide caption

itoggle caption BarackObama.com/YouTube

Peoria, Ill., as seen from across the Illinois River. Jeff Haynes/Reuters /Landov hide caption

itoggle caption Jeff Haynes/Reuters /Landov

A supporter of Republican presidential candidate Mitt Romney holds a vintage campaign poster of Romney's father, George Romney, in Albion, Mich. Justin Sullivan/Getty Images hide caption

itoggle caption Justin Sullivan/Getty Images

Mitt Romney sings the national anthem before speaking at a Tea Party event at the Bakers of Milford Banquet Hall on Thursday in Milford, Mich. Scott Olson/Getty Images hide caption

itoggle caption Scott Olson/Getty Images

Mitt Romney shakes hands with supporters after a town hall meeting at Eagle Manufacturing in Shelby Township, Mich., on Tuesday. Andre J. Jackson/MCT/Landov hide caption

itoggle caption Andre J. Jackson/MCT/Landov

Former Massachusetts Gov. and Republican presidential candidate Mitt Romney addresses supporters during a campaign rally at the Weber Paper Company Monday in Dubuque, Iowa. Chip Somodevilla/Getty Images hide caption

itoggle caption Chip Somodevilla/Getty Images

Supporters seek autographs from Mitt Romney during a campaign event at the Family Table Restaurant Saturday in Le Mars, Iowa. Chip Somodevilla/Getty Images hide caption

itoggle caption Chip Somodevilla/Getty Images

On Jan. 3, Iowans will caucus at 1,774 precincts across the state, in the first contest of the 2012 presidential nominating process. Above, Iowans caucus in 2004 at St. John's United Methodist Church in Des Moines, Precinct 87. Shaun Heasley/Getty Images hide caption

itoggle caption Shaun Heasley/Getty Images

Former Pennsylvania Sen. Rick Santorum speaks Tuesday at the Lincoln Cafe in Belle Plaine, Iowa. Among GOP candidates, Santorum had the state to himself on Tuesday. Chris Carlson/Associated Press hide caption

itoggle caption Chris Carlson/Associated Press