Over all, new voter registration is not only falling short of the big registration surge of the 2008 cycle, but is also running just slightly ahead of the increase before the 2012 caucuses, which was competitive only on the Republican side.

A lower turnout could spell trouble for candidates like Mr. Sanders and Mr. Trump.

The latest voter registration statistics for Iowa, through early January, show that the number of registered voters increased by only about 10,000 voters over the last few months.

The number of newly registered voters who did not affiliate with a party (you can change your registration at the caucus) could be interpreted as positive for candidates like Mr. Sanders or Mr. Trump. They have strong support among voters not registered with a party. But the number increased by almost the same amount as it did in 2012 and 2008, suggesting that the increase in nonpartisan voters was not unusual.

Over all, it is hard to interpret the slow registration pace as anything other than reason to doubt a huge turnout on the Democratic side. (It must be noted that there’s not a tremendous amount of historical data to get a sense of whether increases in registration lead to increases in turnout.)