Although there are clear leaders in the February 3 Democratic caucuses in Iowa — the first Democrat nomination contest of the presidential campaign — many voters still have not completely made up their minds, The Des Moines Register reported on Wednesday.

That should make the 2020 Dem frontrunners nervous — while also boding well for potential dark-horse candidates.

In the last Real Clear Politics poll average reported in December, Mayor Pete Buttigieg of South Bend, Indiana, was the leader — trailed by Sen. Bernie Sanders (I-Vt.) and former Vice President Joe Biden.

But the mayor’s lead was only with 25 percent.

In a poll immediately preceding that, 62 percent of Iowa Democrat voters said they could still be persuaded to vote for someone else.

If a top contender were to emerge out of Iowa, it would not be the first time the early contests had a dramatic impact on a presidential race.

Iowa is not a primary, but a caucus.

A caucus is a vote held in small gatherings in places like homes and reported to local election offices. It is not a traditional vote at a polling place. Thus, the challenge is to a great part organizational and logistical — that is, getting your people to the right place at the right time.

So as much of a test as it is for straight popular support, it also can determine how efficiently the press, donors, and its own staff perceive a campaign.

Related: Julian Castro Drops Out of the 2020 Primary Race

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It sets up the playing field for the New Hampshire primary, a normal primary vote process — where surprises are expected.

In 1968, President Lyndon Johnson won the Dem primary there — but not by enough against new-Left candidate Sen. Eugene McCarthy (D-Minn.). Johnson dropped out of the race.

In 1992, conservative pundit Pat Buchanan beat incumbent President George H.W. Bush in New Hampshire. That win for hard-Right Buchanan presaged much of the Right’s staying at home that year rather than voting for Bush against Democrat Bill Clinton in November.

Combined with the general-election challenge from billionaire entrepreneur Ross Perot, who ran as an independent — Bush lost the race and the presidency.

So while there is a month before the Iowa caucuses and trends are turning into verified support, no candidate or his or her staffs can take anything for granted in that state until the last caucus result is reported.