In 2008, about 40 percent of both Democratic and Republican primary voters (groups with also include a healthy number of independents) made up their minds in the last few days. In 2012, the number was a bit higher.
That bump in last-minute deciders in 2012 boosted Rick Santorum, who had won the Iowa caucuses (barely) a few days beforehand (though this wouldn't become official for more than another week). Among those who'd made up their minds months earlier, Santorum only received 1 percent of the vote. Among those making up their minds only days prior, he won 10 percent -- which was still far lower than the amount of support Mitt Romney earned.
That year, independents also took longer to decide than Republicans. Going into the last weekend, 59 percent of Republican caucus-goers had made up their minds. Only 50 percent of independents had. (When they voted, they largely picked Romney, too.)
That could be more important this year, as there's a bigger split between two of the late-surging Republicans, according to tracking polls. John Kasich gets the support of more independents; Marco Rubio gets the support of more Republicans. If more independents are about to make up their minds, that could help Kasich.
If you're skeptical this all matters, consider the Democrats in 2008. Hillary Clinton came in third in Iowa, and the tide of polling turned against her. New Hampshire voters who decided a month out and right after Iowa preferred Barack Obama. But Clinton had locked up a big advantage among those who decided early -- and voters who decided on primary day gave her a narrow 3-point advantage.
Had the trend of support for Obama continued among those deciding on that day, the state would have been much closer -- or Clinton could have lost.
It didn't. Perhaps because of Clinton's last-minute display of emotion, she held on to those last-minute voters -- and the state.