Charlie Neibergall / AP

Republican presidential hopeful former House Speaker Newt Gingrich speaks at the annual meeting of the Iowa Association of Electric Cooperatives, Thursday, Dec. 1, 2011, in West Des Moines, Iowa.

The once-bursting 2012 Republican presidential field is narrowing to a choice between former Massachusetts Gov. Mitt Romney and former House Speaker Newt Gingrich.

Republican voters have one month before casting the first votes to winnow it to one.

Romney and Gingrich offer striking contrasts in personality, government experience and campaign organization. Gingrich has scant infrastructure in the early voting states. Romney has maintained an organization since his 2008 campaign, especially in New Hampshire.

Both candidates were campaigning on Saturday, Romney in New Hampshire and Gingrich in New York. Neither offered few criticisms of each other. Romney seems content for now to let other rivals such as Ron Paul aim the sharpest barbs at Gingrich, while Gingrich focused more on his vision to bring the U.S. in line with his vision.