In each of the most recent caucuses, for both parties in 2008 and the Republicans in 2012, more than a fifth of Iowans made up their minds about whom to support within the last three days of the campaign, according to exit polls.

In 2008, with fewer candidates to choose from, only 20 percent made up their minds right beforehand, but more than 1 in 10 still made up their minds the day of the caucus. In that weird 2012 election, nearly half of Iowa caucus-goers made up their minds in the last three days, coming to a late decision about whom in the crowded field they would back.

The exit poll shows how that benefited Santorum. Most of Ron Paul's supporters were committed to him for months. Romney's support was even; some people had backed him for a long time and some decided on him at the last minute. Santorum's support is the mirror opposite of Paul's. Two-thirds of the people who decided on him made that decision less than three days before the caucuses ended.

The final gold-standard poll in Iowa released on Saturday — from Selzer and Company — suggested that supporters of both Hillary Clinton and Bernie Sanders were relatively unwavering. There is enough wiggle room, though, that Clinton's slim lead is by no means safe.

Things are less firmly set on the Republican side. A big field, as in 2012. Less fervent support than among the Democrats.

And, just to make things interesting, a late surge from a trailing candidate. That surge is such that the pollsters collecting the data above didn't even ask about Rubio earlier this month.