Presidential candidates have swarmed Iowa’s rolling landscape for more than a year, making their pitch to potential supporters on college campuses, county fairgrounds and in high school gymnasiums. But three weeks before the caucuses usher in the Democratic contest, the battle for the state is wide open. A cluster of candidates, including Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts, along with former Vice President Joe Biden and Pete Buttigieg, the former mayor of South Bend, Indiana, enter the final stretch with a plausible chance of winning Iowa’s caucuses. A poll released Friday by The Des Moines Register and CNN found them all with similar levels of support.