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Kamala Harris has suffered her first drop in the polls since becoming her party's presidential nominee.
A New York Times poll shows Donald Trump leading Ms. Harris 48 percent to 47 percent among likely voters nationwide.
The drop in popularity comes just ahead of a critical televised debate with Donald Trump on Tuesday night.
It is the first major poll to show a drop in support, which the New York Times said could mark the end of a honeymoon period and a "euphoric August" for the vice president.
The poll shows Ms. Harris' lead in the overall average of all polls narrowing to 2.5 points, and in the key swing state of Pennsylvania to just 0.3 points.
Analysts warned that the new poll was an outlier so far. But it is likely to raise fears within Harris' campaign that it could also be a sign of things to come.
"The honeymoon is officially over," Trump spokesman Jason Miller triumphantly told Politico after the poll was released.
The poll also found that 28 percent of likely voters said they wanted to know more about Ms. Harris, while only 9 percent said the same about Mr. Trump. This is a telling result, given that Ms. Harris has been accused of being lax in her policies.
The poll comes as both candidates prepare for Tuesday's debate. Ms. Harris is said to be in a Pittsburgh hotel, preparing intensively on a mock-up stage with an adviser "inhabiting" Mr. Trump in a trademark blue suit and long tie.
Trump is preparing improvisedly, without a stand-in, but with aides who brief him on his policies and ask him questions.
On Sunday, US Transportation Secretary Pete Buttigieg said Ms Harris's main job is "to ensure that Americans understand the different points of view and are reminded that they already agree with her on the issues that matter most to them".
He added: "That's going to be a challenge. It's going to take almost superhuman focus and discipline to deal with Donald Trump. It's not a run-of-the-mill proposition."
On the other hand, Republican Sen. Tom Cotton told CNN that most Americans see the race as a battle between "a president who has served in office and brought good times to America and Kamala Harris, a liberal from San Francisco, who has brought to America exactly what you see in San Francisco."
Cotton dismissed George Bush's Vice President Dick Cheney's switch to the Democrats as a "push for electoral politics." What mattered, he added, was the candidates' track records.
Leading US Republicans urged Trump to stick to a disciplined policy on Tuesday, warning that he could harm himself if he makes personal attacks.
Sen. Lindsey Graham wrote in a New York Times op-ed last week that "every day the candidates trade insults is a good day for Harris]because it is one less day she has to defend the failings of the Biden-Harris administration."
"I think - I pray - he can be disciplined," Republican strategist Tricia McLaughlin told Politico. "If Trump feels like he's backed into a corner and it feels like it's three against one, that could be a problem."
After an extended tug-of-war over debate rules, Trump has accused the ABC News host of being "dishonest" and "the meanest."
Former House Speaker Newt Gingrich said he expected Ms Harris to "be very, very aggressive and try to provoke him, get very angry and get personal and try to belittle him".
He added: "I think, I hope, that he will be a man who has been a real president - while she has been sort of a semi-vice president - and a man who knows all the world leaders, and a man who has been through an enormous amount, and will just be calm and steadfast and abide by the real differences."
Trump, however, recently disagreed with that approach, saying he has a "right to make personal attacks" on Ms. Harris and joking at a rally that he should "fire" his advisers who had told him to refrain from personal insults against his opponent.
Trump is now a veteran of seven presidential debates, while Harris has debated just once - when she blasted Joe Biden for his support for racially motivated school busing during the race to become the Democratic nominee in 2019. She was in turn blasted by Trump's debate team adviser and former Democratic congresswoman Tulsi Gabbard for her record as California's attorney general.
Former Trump adviser Kellyanne Conway said the onus would fall on Ms Harris in the 90-minute debate. "There is no raw material when it comes to Donald Trump. It's all said and done."
Conway warned that if Ms. Harris were to repeat "the same tired slogans," she would come across as "a cable news pundit, not a commander in chief."
He added: "America is still getting to know her, and while that has helped her in the short term, it is a risk for her in the long term."
The New York Times poll shows that what some Democratic advisers warned would be a "sugar rush" of support for Ms. Harris could be waning. The poll also suggests a wide gender divide, with women favoring Harris by 11 points (53 percent to 42 percent) and men favoring Trump by 17 points (56 percent to 39 percent). Ms. Harris also holds a lead among black and Hispanic voters, and younger voters favor her by eight points.
James Johnson, co-founder of polling firm JL Partners, said Ms. Harris's standing had been negatively impacted by Robert F. Kennedy Jr.'s decision to withdraw from the race last month.
Mr Johnson said: "The honeymoon is over for Harris.
"Harris peaked a week before the DNC. There was a sense of relief when she took over from Biden and she was able to motivate people who were not impressed with him, but since RFK dropped out of the race, which helped to bolster voters for Trump and his position, [Ms Harris flagging in the polls] is a trend that has been going on for some time now.
"There is a desire to see more detail from her. The fact that she is not doing interviews is a concern for some voters."
According to Johnson, the debate between the two candidates on Tuesday will be a crucial moment in the race.
"The debates are really important," he said. "It all depends on Tuesday."
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