Pete Buttigieg has jumped 7 percentage points since a June Suffolk/USA TODAY poll.

Joe Biden was by far the candidate seen as doing worse than expected in last week's debate.

Support for impeaching President Donald Trump has risen significantly since the June poll.

It's a new three-way race in Iowa.

Pete Buttigieg, the mayor of South Bend, Indiana, who was initially seen as a long-shot presidential contender, has surged within striking distance of former vice president Joe Biden and Massachusetts Sen. Elizabeth Warren in the first-in-the-nation Iowa caucuses, a Suffolk University/USA TODAY Poll finds.

Biden, long viewed as the Democratic frontrunner, is faltering in the wake of a debate performance last week that those surveyed saw as disappointing.

The poll, taken Wednesday through Friday, put Biden at 18%, Warren at 17% and Buttigieg at 13% among 500 likely Democratic caucusgoers.

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Those standings reflect significant changes since the Suffolk/USA TODAY poll taken in Iowa at the end of June, when Biden led Warren by double digits and Buttigieg trailed at a distant 6%. California Sen. Kamala Harris, who was then in second place after a strong showing in the first Democratic debate, has plummeted 13 percentage points and is now in a three-way tie for sixth. Vermont Sen. Bernie Sanders earned 9% support, the same number as in the June poll.

"Iowa is unquestionably up for grabs," said David Paleologos, director of the Suffolk Political Research Center. Buttigieg "has found a lane and is accelerating toward the front of the pack, surpassing Bernie Sanders. All of this is happening while the number of undecided voters continues to grow as Democratic caucusgoers pause to reevaluate the changing field."

Paleologos on the Poll:Buttigieg gets an Iowa boost on strength of his debate performance

Buttigieg as mayor:How Buttigieg's 'beta city' approach highlights his differences with fellow candidates

After the debate:Joe Biden disputes Warren's place as frontrunner

The number of caucusgoers who say they are undecided has spiked 8 points since June to 29%. Among those who have a preferred candidate, nearly two-thirds (63%) say they might change their minds before the caucuses.

Among the second tier of candidates, activist Tom Steyer was at 3%. Three other candidates also reached 3% because of rounding: Hawaii Rep. Tulsi Gabbard, Harris and Klobuchar.

The Iowa caucuses, now 105 days away, historically have had an outsize influence in propelling the winner into the primaries that follow in New Hampshire and South Carolina – and in creating sobering challenges for those who do worse than expected.

At 37, Buttigieg is the youngest contender in the field, and he is the first openly gay candidate to seek the Democratic Party's presidential nomination. He has gained ground through strong performances in the Democratic debates: Among those surveyed who watched the debate last Tuesday, 4 in 10 said Buttigieg was the candidate who did better than they expected.

An additional 3 in 10 cited Minnesota Sen. Amy Klobuchar as doing better than expected, although so far that hasn’t translated into support. She is just below 3% in the poll.

Biden was by far the candidate seen as doing worse than expected in the debate, cited by 1 in 4 poll respondents.

Among debate-watchers only, Buttigieg held a narrow lead in the poll, at 19%. Biden and Warren were tied at 17%.

There were other signs of a friendly political landscape for Buttigieg and Warren in the poll. They led the field as the second choice of respondents; Warren was picked by 22% and Buttigieg by 14%. That could be important if and when other candidates drop out of the race, or if other candidates fail to reach the 15% threshold for delegates in the caucuses and caucusgoers decide to move to another contender.

The list:Who is running for president in 2020? An interactive guide

In the survey, Democrats who hadn’t named Warren as their first or second choice were asked why they weren’t supporting her. In the open-ended question, the three most frequent responses were: that they didn’t know enough about her; that they might support her; and that they were undecided. Those are all factors that the Massachusetts senator could address in her campaign.

Others said they didn’t agree with her on issues. About 5% said she wasn’t electable or couldn’t defeat President Donald Trump.

When the same question was asked in the June survey about why Democrats hadn’t named Biden as their first or second choice, the most frequent response was that he was too old. Others said they wanted fresh ideas or that it was time to “pass the torch” to a new generation.

Support for impeaching President Trump has risen significantly. In the June poll, before disclosures about White House contacts with Ukraine prompted the U.S. House to launch a formal impeachment inquiry, 41% said it was very important that the Democratic nominee support impeachment. Now that number has risen to 52%.

The telephone poll has a margin of error of plus or minus 4.4 points.