The first Democratic debate reshaped the presidential field in Iowa as support surged for California Sen. Kamala Harris, undercutting the standing of Vermont Sen. Bernie Sanders and raising questions about the solidity of former Vice President Joe Biden's front-runner status.

In a new Suffolk University/USA TODAY Poll, Biden continues to lead the field, backed by 24% of those who say they are likely to attend the Democratic caucuses in Iowa that will open the presidential contests next year. Harris jumped to second place, at 16%, leapfrogging over Sanders, whose support sagged to single digits. At 9%, he finished fourth, behind Massachusetts Sen. Elizabeth Warren at 13%.

The new standings are hardly set in stone. Twenty-one percent are undecided. Six of 10 who have decided say they might change their mind before the caucuses. One in four say their minds are firmly made up. The second set of debates, scheduled for the end of the month in Detroit, could upend the horse race again.

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The survey has some sobering findings for Biden, a familiar figure in the state. His level of support didn't change from the Des Moines Register/Mediacom/CNN Iowa Poll taken last month, but among those who watched him debate, more than four in 10 say he did worse than they expected. Among all of those surveyed, he is the second choice of 11%.

Harris saw her support more than double compared with the poll in June, to 16% from 7%. She is the second choice of 17% – a telling measure of the potential breadth of a candidate's support. When first and second choices are combined, Biden only narrowly edges Harris, 35% to 33%.

The poll of 500 likely Democratic caucusgoers in Iowa, taken Friday through Monday, has a margin of error of plus or minus 4.4 percentage points. The opening back-to-back debates, featuring 10 contenders each night, were held last Wednesday and Thursday in Miami.

"To win in Iowa, you have to be able to woo the supporters of other candidates who drop out or that don't reach the 15% threshold at the caucus," says David Paleologos, director of Suffolk University's Political Research Center. "The poll tells us that candidates like Harris, Warren and (Pete) Buttigieg poll better than Biden and Sanders in this regard, and that sets the stage for a new face exceeding expectations in Iowa."

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For Sanders, who lost the 2016 Iowa caucuses to Hillary Clinton by less than a point, the new poll has warning flags. His support is 7 points lower than in the Iowa Poll taken in June, when he finished second to Biden. When first and second choices are combined, Sanders finishes fifth – trailing Biden, Harris and Warren by double digits and Buttigieg by single digits.

Buttigieg, the mayor of South Bend, Indiana, shows lower support than in June. Down 8 points from the Iowa Poll, he finishes fifth, at 6%.

A national poll taken by CNN after the debate and released Monday showed some trends similar to those in Iowa. Across the country, the Democratic race tightened, and Biden's lead over the field shrank to 5 points, to 22%. Support for Harris and Warren significantly increased, to 17% and 15%, respectively. Sanders was fourth at 14%.

Both the Iowa and national polls show the power of the televised debates to introduce lesser-known candidates to voters, helping them to break out of a big field. Even so, three of those who participated in the debates – New York Mayor Bill de Blasio, Ohio Rep. Tim Ryan and California Rep. Eric Swalwell – failed to attract a single supporter in the Iowa survey.

Harris, who has spent limited time campaigning in the state, surprised many viewers by how well she did. More than half of those who watched her debate say she did better than expected; fewer than 5% say she did worse than expected. Former Housing secretary Julian Castro, New Jersey Sen. Cory Booker, Warren and Buttigieg also significantly outdid expectations.

Biden and Sanders were by far the biggest disappointments. By 5-1, 41%-8%, those who watched both nights of the debate say Biden did worse than they had expected. By 6-1, 23%-4%, those who watched both nights say Sanders did worse than expected.

The poll tried to explore Biden's strengths and weaknesses.

Among those who support Biden, 88% say they would still vote for the Democratic candidate in the general election if he wasn't the nominee. Five percent say they would vote for President Donald Trump instead, 4 percent for a third-party candidate. Biden described himself as the Democratic contender who would be best able to win support from Trump voters and defeat the president's bid for a second term.

Among those who didn't choose Biden as either their first or second choice, they tend to cite a feeling that his time has passed as the main reason. In response to an open-ended question, 15% mention his age or say they want someone younger; 7% say they want "fresh ideas;" another 7% say it's time to "pass the torch."

In a favorable sign for Biden, among those who say their minds are "firmly made up," 38% back Biden and 19% Sanders. If their supporters stick with them, that could give them resiliency through the ups and downs of the campaign ahead.

See all the details:Suffolk/USA TODAY Poll in Iowa

Biden has sometimes struggled to explain his actions during his 36 years as a U.S. senator from Delaware, including his handling as Judiciary Committee chairman of accusations of sexual misconduct by Anita Hill against Supreme Court nominee Clarence Thomas. During last week's debate, Harris castigated him for his opposition to school busing. He also has drawn fire for boasting about his ability in past decades to work with segregationist senators.

Those surveyed say the most important issue affecting their support is health care, named by nearly three in 10. Other top issues include climate change, immigration and income inequality. Two issues that often determine presidential elections, the economy and national security, score only in single digits.

There is a relatively close divide on views of reparations for slavery, of making either payments to descendants of enslaved Africans or investments in aggrieved communities. Thirty-nine percent support the idea; 43% oppose it.

Solid majorities say it is "very important" to them that Democrats nominate a presidential candidate who supports higher taxes on the very wealthy (64%) and "Medicare for All" (57%). Impeaching Trump? Four in 10, 41%, say that is "very important" to them.

Those surveyed care more about victory than ideology. Sixty percent say the most important thing for Democrats to do is to nominate a presidential candidate who can defeat Donald Trump on Election Day. That is close to double the 34% who say the most important thing is to nominate a candidate who reflects their priorities.

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