After months of campaigning, the Iowa caucuses tonight mark a new phase in the race for the White House, and new poll numbers show a very close contest on both sides of the aisle.

A recent Des Moines Register/Bloomberg poll shows Donald Trump five points ahead of Sen. Ted Cruz, but with many other GOP contenders in single digits, gathering up other candidates' supporters could be crucial.

The same poll showed Hillary Clinton with a three-point lead over Sen. Bernie Sanders -- that's a statistical dead heat. But if former Maryland Gov. Martin O'Malley supporters can't bring together 15 percent of the vote at any given caucus site, Clinton and Sanders will fight for those votes to push themselves over the top.

Unlike the primaries where voters can cast their ballots throughout the day, the caucuses begin in Iowa at 7 p.m. Central Standard Time, 8 p.m. on the East Coast.