Caucuses in the U.S. state of Iowa open Tuesday marking the beginning of the Republican primary season.

Former Massachusetts Governor Mitt Romney continues to lead polls followed by Senator Ron Paul of Texas and former Pennsylvania Senator Rick Santorum.

Behind the front-runners in the Hawkeye State are former House Speaker Newt Gingrich, Texas Governor Rick Perry, and Minnesota Congresswoman Michele Bachmann. All three are appealing for last-minute votes in Iowa ahead of the caucuses.

According to a survey by Gallup, competition to earn the Republican presidential nomination is the fiercest in decades as the poll leader has changed seven times so far.