A surprising Iowa poll that shows Vice President Kamala Harris with a slight lead in the state over former President Donald Trump has thrown a "monkey wrench" into the homestretch of the campaign, according to Rick Klein, ABC News Washington bureau chief and political director.

With a day to go before Election Day, a Des Moines Register/Mediacom Iowa poll conducted by renowned Iowa pollster J. Ann Selzer showed Harris with a 47% to 44% advantage among likely voters over Trump, who scored solid victories in the Hawkeye State in 2016 and 2020 presidential races.

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The poll, released on Saturday, follows a Des Moines Register/Mediacom poll in September that gave Trump a four-percentage-point lead over Harris in Iowa, and one in June that had him with an 18-point lead over President Joe Biden, who was then the presumptive Democratic nominee.

"This really threw a monkey wrench into everyone's pre-election analysis," Klein said on "ABC News Live" Sunday night. "This is a poll in Iowa – yes, Iowa – that has Kamala Harris leading by three points."

Trump took to social media to attack both the Des Moines Register poll and Selzer, who has overseen the Register's polls since 1987 and whom Klein described as a "very reputable pollster."

“No President has done more for FARMERS, and the Great State of Iowa, than Donald J. Trump,” Trump wrote in a post on his Truth Social network on Sunday morning. “In fact, it’s not even close! All polls, except for one heavily skewed toward the Democrats by a Trump hater who called it totally wrong the last time, have me up, BY A LOT."

In a Des Moines Register poll released just before the 2020 election, Selzer found Trump leading Biden 48% to 41%. Trump went on to win Iowa 53% to 45%.

"It's hard for anybody to say they saw this coming," Selzer told the Des Moines Register of the latest poll. "She [Harris] has clearly leaped into the leading position."

"Age and gender are the two most dynamic factors that are explaining these numbers," Selzer added.

With just hours remaining in the race, both candidates are working to sway undecided voters and make their final pitches in their bids for the White House. Harris was campaigning Monday in the swing state of Pennsylvania, while Trump was making campaign stops in North Carolina and Pennsylvania.

More than 78 million people had already voted as of Monday afternoon, including a record number of in-person votes, according to the University of Florida Election Lab.

"There aren’t many people who think Donald Trump is going to suddenly lose the state that he won by eight points, but it might indicate some of the weakness he has with voters in other states, including Wisconsin and Michigan," Klein said of the Des Moines Register's latest poll.

An ABC News/Ipsos poll released Saturday showed Harris with an overall three-point advantage over Trump among likely voters nationwide, 49% to 46%.

"The polls haven’t moved since Kamala Harris got in the race," Klein said Monday on ABC's "Good Morning America," referring to the ABC News/Ipsos poll, which shows support for either candidate hasn't changed significantly since then.

Klein also cited the latest New York Times/Siena College poll showing Harris making up ground in some of the seven battleground states. The poll found that Harris is up in Nevada by three points, up in North Carolina and Wisconsin by two points and ahead by one point in Georgia. The poll found Trump was ahead in Arizona by four points and that the race was tied in Pennsylvania and Michigan.

As far as the gender gap goes, the ABC News/Ipsos poll found women overall going for Harris by 11 percentage points, while women registered as independent were supporting Harris by 18 percentage points and women under 30 were going for the vice president by 40-plus points.

The ABC News/Ipsos poll found Trump leading overall among men by five percentage points, while men registered as independent were going for Trump by four percentage points and men under 30 were siding with Trump by five percentage points.

Klein said the ABC News/Ipsos poll also found that 74% of Americans say the country is on the wrong track, including half of Harris supporters and 98% of Trump supporters.

"Wild numbers," Klein said. "And those are the kinds of things that are just typically brutal for an incumbent party, but, of course, this is an unusual year in so many ways."