Kamala Harris is more popular than Donald Trump, according to the first national poll carried out since Joe Biden stood down.
Research by Ipsos and Reuters published on Tuesday shows Ms Harris leads Trump by two points nationally, by 44 per cent of the vote to 42 per cent.
A previous poll by the same firm previously found that Trump led Joe Biden by two points.
The survey suggests Ms Harris has a fighting chance of overturning Trump’s lead in the swing states ahead of the presidential election on November 5.
Mr Biden stood down his campaign on Sunday, after weeks of concern from Democrats about his age and health.
The vice-president used her first rally since taking over the Democratic presidential campaign to attack Trump for his criminal convictions, labelling him a liar, a fraudster and a cheat.
Speaking in Milwaukee, the Ms Harris emphasised her record as a state prosecutor to draw a comparison between herself and Trump.
“I was elected Attorney General of the State of California and I was a corporate prosecutor before that, and in those roles I took on perpetrators of all kinds,” she said.
“Predators who abused women, fraudster who ripped off consumers, and cheaters who broke the rules for their gain.
“So hear me when I say I know Donald Trump’s type.”
Meanwhile, Trump said that he is ready to debate Ms Harris “more than once”, after recently describing her an “easier” opponent than Mr Biden.
Trump did not commit to appearing at the next scheduled debate in September on ABC, saying he was “not thrilled” about the choice of network and may want another to air the debate.
But Trump said he still wants to debate Ms Harris. “I think debating’s important for a presidential race,” he said. “You sort of have an obligation to debate.”