(Bloomberg) -- The first debate shuffled the Democratic presidential race, vaulting Kamala Harris into second place and highlighting vulnerabilities for front-runner Joe Biden.Meanwhile, Elizabeth Warren continues to rise and 2016 runner-up Bernie Sanders is slipping as voters search for alternatives in the historically large field of candidates.That’s the conclusion from four polls taken since last week’s debate that also show those four candidates clearly make up the top tier at this stage of the Democratic nomination race. None of the other 20 candidates breaks into double-digit support, including Pete Buttigieg, who’s gotten plenty of media attention and managed to raise an eye-popping $24.8 million in the second quarter.The shifts are a snapshot in time seven months before the contest officially begins in Iowa, with 11 more debates before the party convention next July. The new surveys of Democratic voters indicated that support for various candidates remains very fluid, and history shows early polls frequently don’t match final results.A Quinnipiac national poll released Tuesday found Biden slipping to 22%, just two points ahead of Harris at 20%. Warren was next with 14%, ahead of Sanders with 13%.Notably, the former vice president’s lead among black voters was down to just four points, at 31% to Harris’s 27% with this key constituency that has picked the winner in all five contested Democratic presidential primaries since 1992.A CNN national poll found Biden slipping 10 points in one month — to 22%. Behind him was California Senator Harris with 17% (a 9-point boost) and Massachusetts Senator Warren with 15% (an 8-point gain), and Vermont Senator Sanders with 14% (a 4-point drop). Next came Buttigieg with 4% and Beto O’Rourke and Cory Booker with 3% each.Iowa SurveyThe same results were reflected in Iowa, where caucuses formally kick off the nomination contest next February and where Biden, Harris and Sanders are campaigning this week. A Suffolk/USA Today poll of Iowa Democrats found Biden leading with 24%, behind a rising Harris at 16%, and Warren at 13%; Sanders came fourth with 9%.The surveys signal trouble for Sanders. CNN’s poll showed him with the highest unfavorable ratings of any Democrat among the general public and party voters. And his single-digit showing in the Iowa survey is a troubling sign for someone who’s well-known among the state’s caucus-goers and finished a mere 0.25 point behind Hillary Clinton there in 2016.A HuffPost/YouGov national survey found that Democratic voter opinions of Warren and Harris improved the most since the debate, while perceptions of Biden worsened more than they improved, and Sanders barely changed with a 2-point net improvement.The bright spot for Biden is that he continues to outperform the field on “electability” perceptions. The CNN survey found that 43% of Democrats say he “has the best chance to beat Donald Trump” next fall, followed by 13% for Sanders and 12% each for Harris and Warren.The surveys also established a middle tier of candidates polling behind the front-runners and ahead of the bulk of the two dozen who are running.Buttigieg, the mayor of South Bend, Indiana, is struggling despite his impressive fundraising. And O’Rourke, a former U.S. representative who began the race with high expectations after a strong showing in last year’s Texas Senate race, is languishing after a sub-par debate performance.Buttigieg was polling at 0% with black voters in the new Quinnipiac poll.The good news for them is that polls this early tend to be off the mark. A CNN poll at this stage of the 2016 Republican primary found Jeb Bush leading with 19%, ahead of Trump with 12%; Senator Ted Cruz, who finished as the runner-up, was just 10th with 3%.To contact the reporter on this story: Sahil Kapur in Washington at skapur39@bloomberg.netTo contact the editors responsible for this story: Joe Sobczyk at jsobczyk@bloomberg.net, Wendy BenjaminsonFor more articles like this, please visit us at bloomberg.com©2019 Bloomberg L.P.