DES MOINES, Iowa (CBS News) – This week marks one year until the next presidential election, and with the road to Campaign 2020 running through Iowa first, several Democratic hopefuls made the political rounds there last weekend.

Polling released over the weekend shows that nationally this is still a three-way race between Senators Bernie Sanders and Elizabeth Warren and former Vice President Joe Biden, reports CBS News’ Ed O’Keefe in Des Moines.

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In Iowa, where voters are paying more attention and getting more attention from the candidates — Indiana Mayor Pete Buttigieg has slipped into third place.

Buttigieg irritated some over the weekend by alluding to the idea that it was a two-man race between himself and Warren. He pulled back on that idea Sunday but not before some candidates fired back.

California Sen. Kamala Harris spoke out about the race from Cedar Rapids.

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“Well I just think that’s just naive for him to think that at this point that the fate of this election has been determined,” she said. “Just look at history. He might need to review past elections to know that what’s happening right now is not necessarily determinative of the outcome.”

Now Harris is the first of what we expect to be several candidates who aren’t in the top tier to have to downsize their campaigns in the coming weeks as they focus in on the race here in must-win Iowa.

She has laid out most of her staff in New Hampshire, vowing to do well enough in Iowa to continue on. That’s not a sign that the end is near, it’s just a good example of recalibrating.

Back in 2003, when he was in single digits, John Kerry did much the same in Iowa before wining the caucus and the nomination. And in 2007, with a skeleton staff, John McCain did it in New Hampshire, before turning his campaign around and winning his party’s nomination.

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